Contrail parameters

ABSTRACT

A method of determining one or more fuel characteristic of an aviation fuel suitable for powering a gas turbine engine of an aircraft is disclosed. The method includes: determining, during use of the gas turbine engine, one or more contrail parameters related to contrail formation by the gas turbine engine. The determining of the one or more contrail parameters includes performing a sensor measurement on a region behind the gas turbine engine in which a contrail is or can be formed. The method also includes determining one or more fuel characteristics of the fuel based on the one or more contrail parameters. A fuel characteristic determination system, a method of operating an aircraft and an aircraft are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.17/853,405, filed on 29 Jun. 2022, which claims priority from UnitedKingdom Patent Application Number 2118640.8, filed on 21 Dec. 2021. Theentire contents of each of these applications are incorporated herein byreference in their entirety.

BACKGROUND Field of the Disclosure

The present disclosure relates to methods of determining one or morefuel characteristics of an aviation fuel for powering a gas turbineengine of an aircraft, and fuel characteristic determination systems forthe same. The present disclosure further relates to methods of operatingan aircraft, for example according to the determined fuelcharacteristics, and an aircraft having a fuel characteristicdetermination system and a control system. The present disclosurefurther relates to a method of generating a maintenance schedule for anaircraft, a maintenance schedule generation system, and a method ofmaintaining an aircraft.

DESCRIPTION OF THE RELATED ART

There is an expectation in the aviation industry of a trend towards theuse of fuels different from the traditional kerosene-based jet fuelsgenerally used at present. These fuels may have differing fuelcharacteristics, for example having either or both of a lower aromaticcontent and sulphur content, relative to petroleum-based hydrocarbonfuels.

In order to take advantage of the different properties of these fuelsthere is a need for methods of determining fuel characteristics, eitheron board an aircraft during its operation, or as it is being refuelled.Based on this determination the aircraft, and more specifically a gasturbine engine used to power it, can be operated or maintainedaccordingly. This may provide performance benefits and/or environmentalbenefits by making better use of the characteristics of the fuel presentonboard the aircraft.

SUMMARY

According to a first aspect there is provided a method of determiningone or more fuel characteristics of an aviation fuel suitable forpowering a gas turbine engine of an aircraft, the method comprising:

-   -   exposing the surface of a piezoelectric crystal to the fuel;    -   measuring a vibration parameter of the piezoelectric crystal;        and    -   determining one or more fuel characteristics of the fuel based        on the vibration parameter.

The inventors have determined that the vibrational properties of apiezoelectric crystal varies according to the characteristics of fuel towhich it has been exposed. By measuring a vibrational parameter of thecrystal, characteristics of the fuel to which it has been exposed maytherefore be determined. For example, deposits may be formed on thecrystal surface by exposure to fuel. By measuring a vibration parameterof the crystal (e.g. a resonant frequency) the amount surface depositionon the crystal surface can be determined, and the characteristics of thefuel which caused those deposits determined.

The vibration parameter may be indicative of a surface deposition formedon the surface of the piezoelectric crystal which has been exposed tothe fuel.

Measuring the vibration parameter may comprise measuring a change in avibrational mode of the piezoelectric crystal.

The one or more fuel characteristics determined may include ahydrocarbon distribution of the fuel.

The one or more fuel characteristics determined may include any one ormore of (i) the percentage of sustainable aviation fuel in the fuel;and/or (ii) the aromatic hydrocarbon content of the fuel.

The one or more fuel characteristics may include any one or more of: (i)an oxygen content of the fuel; (ii) a thermal stability of the fuel;and/or (iii) a coking level of the fuel.

The fuel may be exposed to the surface of the piezoelectric crystalduring use of the gas turbine engine.

The method may comprise exposing fuel within a fuel system of the gasturbine engine to the surface of the piezoelectric crystal. Preferablythe fuel may be exposed to the piezoelectric crystal after the fuel hasbeen heated by a heat exchanger of the gas turbine engine.

Fuel within, sampled from, or being delivered to a fuel tank of theaircraft may be exposed to the surface of the piezoelectric crystal.

Measuring the vibration parameter may comprise exposing thepiezoelectric crystal to fuel outside of the aircraft during a fuelloading process in which a fuel tank or tanks of the aircraft are loadedwith fuel.

According to a second aspect, the present application provides a fuelcharacteristic determination system for determining a fuelcharacteristic of an aviation fuel suitable for powering a gas turbineengine of an aircraft, the system comprising:

-   -   a sensor comprising a piezoelectric crystal, a surface of the        piezoelectric crystal adapted to be exposed to the fuel, the        sensor being arranged to measure a vibration parameter of the        piezoelectric crystal; and    -   a fuel characteristic determination module arranged to determine        one or more fuel characteristics of the fuel based on the        vibration parameter.

The vibration parameter may be indicative of a surface deposition formedon the surface of the piezoelectric crystal (which has been exposed tothe fuel).

The sensor may be arranged to measure a change in a vibrational mode ofthe piezoelectric crystal in order to measure the vibrational parameter.

The one or more fuel characteristics determined may include any one ormore of (i) a hydrocarbon distribution of the fuel; (ii) a percentage ofsustainable aviation fuel in the fuel;

-   -   (iii) an aromatic hydrocarbon content of the fuel; (iv) an        oxygen content of the fuel; (v) a thermal stability of the fuel;        and/or (vi) a coking level of the fuel.

The fuel may be exposed to the surface of the piezoelectric crystalduring use of the gas turbine engine.

The piezoelectric crystal may be adapted for exposure to fuel within afuel system of the gas turbine engine. Preferably, the piezoelectriccrystal may be adapted to be located at a position downstream (in thedirection of fuel flow) of a heat exchanger of the gas turbine engine.

The piezoelectric crystal may be adapted for exposure to fuel that iswithin, sampled from, or being delivered to a fuel tank of the aircraft.The piezoelectric crystal may be adapted for exposure to fuel outside ofthe aircraft, the fuel being loaded onto the aircraft during a fuelloading process. The piezoelectric crystal may be provided in a fuelloading line or fuel storage vessel of a refuelling system.

According to a third aspect, the present application provides a methodof operating an aircraft having a gas turbine engine, the methodcomprising:

-   -   determining one or more fuel characteristics using the method of        the first aspect; and    -   operating the aircraft according to the one or more fuel        characteristics.

Operating the aircraft according to the one or more fuel characteristicsmay comprise:

-   -   a) modifying a control parameter of the aircraft, preferably a        control parameter of the gas turbine engine, in response to the        one or more fuel characteristics; and/or    -   b) providing a fuel having different fuel characteristics (to        those of the fuel for which the fuel characteristics have been        determined) during refuelling of the aircraft.

According to a fourth aspect, the present application provides anaircraft comprising the fuel characteristic determination system of thesecond aspect, the aircraft further comprising a control system arrangedto control operation of the aircraft according to the one or more fuelcharacteristics determined by the fuel characteristic determinationsystem.

According to a fifth aspect, the present application provides a methodof determining one or more fuel characteristics of an aviation fuel forpowering a gas turbine engine of an aircraft, the method comprising:

-   -   exposing the surface of a sensor component formed from a nitrile        seal material to the fuel;    -   measuring a swell parameter of the seal material; and    -   determining one or more fuel characteristics of the fuel based        on the swell parameter.

The inventors have determined that a swell parameter of a seal material,such as a nitrile seal, may be used to determine the characteristics ofa fuel to which the seal material has been exposed. The inventors havedetermined that the degree to which the nitrile material swells onexposure to the fuel is dependent on the characteristics of the fuel,and can be used as a sensor to determine such characteristics.

Measuring the swell parameter may comprise measuring the expansion orcontraction of the sensor component as a result of exposure to the fuel.

Measuring the expansion or contraction of the sensor component maycomprise measuring a change in physical size of the sensor component, ormeasuring a force applied to a gauge by the sensor component.

The one or more fuel characteristics determined may include ahydrocarbon distribution of the fuel.

The one or more fuel characteristics determined may include any one ormore of: (i) a percentage of sustainable aviation fuel in the fuel; (ii)an aromatic hydrocarbon content of the fuel; and/or (iii) acycloparaffin content of the fuel.

The method may further comprise generating an alert signal if the swellparameter is outside of an alert threshold. The alert signal may begenerated if the swell parameter is outside of a safe operating range.

The fuel may be exposed to the surface of the sensor component duringuse of the gas turbine engine.

The fuel may be exposed to the surface of the sensor component within afuel system of the gas turbine engine. The fuel may be exposed to thesurface of the sensor component in a bleed line of the fuel system ofthe gas turbine engine.

Fuel within, sampled from, or being delivered to a fuel tank of theaircraft may be exposed to the surface of the sensor component.Measuring the swell parameter may comprise exposing the sensor componentto fuel outside of the aircraft during a fuel loading process in which afuel tank or tanks of the aircraft are loaded with fuel. The sensorcomponent may be provided in a fuel loading line or fuel storage vesselof a refuelling system for the aircraft.

According to a sixth aspect, the present application provides a fuelcharacteristic determination system for determining one or more fuelcharacteristics of an aviation fuel for powering a gas turbine engine ofan aircraft, the system comprising:

-   -   a sensor component formed from a nitrile seal material, a        surface of the sensor component adapted to be exposed to the        fuel;    -   a sensor arranged to measure a swell parameter of the seal        material; and    -   a fuel characteristic determination module arranged to determine        one or more fuel characteristics of the fuel based on the swell        parameter.

The sensor component formed from the seal material may be arranged to befixedly mounted relative to a gauge. The gauge may be arranged to detectmovement of the sensor component due to expansion or contraction.

The gauge may be arranged to detect a change in physical size of thesensor component.

The gauge may be arranged to detect a pressure exerted by the sensorcomponent resulting from its expansion or contraction.

The one or more fuel characteristics determined by the fuelcharacteristic determination module may include a hydrocarbondistribution of the fuel.

The one or more fuel characteristics determined by the fuelcharacteristic determination module may include any one or more of: (i)a percentage of sustainable aviation fuel in the fuel; (ii) an aromatichydrocarbon content of the fuel (F); and/or (iii) a cycloparaffincontent of the fuel.

The fuel characteristic determination system may be further arranged togenerate an alert signal if the swell parameter is beyond an alertthreshold.

The sensor component may be adapted to be located on board the aircraft.The sensor component may be arranged so that fuel is exposed to itssurface during operation of the gas turbine engine.

The sensor component may be adapted for exposure to fuel within, sampledfrom, or being delivered to a fuel tank of the aircraft. The sensorcomponent may be adapted for exposure to fuel outside of the aircraft,the fuel being loaded onto the aircraft during a fuel loading process.The sensor component may be provided in a fuel loading line or fuelstorage vessel of a refuelling system for the aircraft.

According to a seventh aspect, the present application provides a methodof operating an aircraft having a gas turbine engine, the methodcomprising:

-   -   determining one or more fuel characteristics using the method of        the fifth aspect; and    -   operating the aircraft according to the one or more fuel        characteristics.

Operating the gas turbine engine or the aircraft according to the one ormore fuel characteristics may comprise:

-   -   a) modifying a control parameter of the aircraft, preferably a        control parameter of the gas turbine engine, in response to the        one or more fuel characteristics; and/or    -   b) providing a fuel having different fuel characteristics during        refuelling of the aircraft.

Modifying a control parameter of the aircraft may comprise modifying acontrol parameter which controls a selection of fuel to be supplied tothe gas turbine engine from different fuel sources on board the aircraftso as to provide fuel having a different fuel characteristic (from thatof the fuel determined).

Providing fuel having a different fuel characteristic may comprise anyone or more of i) providing fuel with a relatively higher aromaticcontent; ii) providing fuel with a relatively lower SAF content; and/oriii) providing fossil kerosene fuel.

According to an eighth aspect, the present application provides anaircraft, comprising:

-   -   a gas turbine engine;    -   a fuel system comprising one or more fuel tanks arranged to        contain fuel for supply to the gas turbine engine, the fuel        system comprising one or more seals, the seals being exposed at        least partly to the fuel;    -   a detection device located within the fuel system and comprising        a sensor component made of the same material as the one or more        seals, the detection device being arranged to measure a swell        parameter of the seal material.

By measuring the level of swell of a sensor component made from the samematerial as one or more seals provided in the aircraft fuel system thebehaviour of those seals as a result of exposure to the fuel can beindirectly determined. This may provide an indication that an inadequate(e.g. too little) amount of seal swell has, or is, occurring to providesufficient sealing performance.

The detection device may be arranged to measure the expansion orcontraction of the sensor component as a result of exposure to the fuel.

The detection device may be arranged to measure a change in physicalsize of the sensor component. The detection device may be arranged tomeasure a force applied to a gauge by the sensor component.

The aircraft may further comprise a fuel characteristic determinationmodule arranged to determine one or more fuel characteristics of thefuel based on the swell parameter.

The one or more fuel characteristics may include any one or more of: (i)a hydrocarbon distribution of the fuel; (ii) a percentage of sustainableaviation fuel in the fuel; (iii) an aromatic hydrocarbon content of thefuel; and/or (iv) a cycloparaffin content of the fuel.

The detection device may be arranged to generate an alert signal if theswell parameter is beyond an alert threshold.

According to a ninth aspect, the present application provides a methodcomprising:

-   -   exposing one or more seals of a fuel system of an aircraft to        fuel within the fuel system, the aircraft having a gas turbine        engine supplied by fuel by the fuel system exposing a sensor        component, made from the same material as the one or more seals,        to the fuel, the component being located within the fuel system;        and    -   measuring a swell parameter of the seal material.

The method may further comprise determining one or more fuelcharacteristics of the fuel based on the swell parameter.

The method may further comprise generating an alert signal if the swellparameter is beyond an alert threshold.

According to a tenth aspect, the present application provides a methodof operating an aircraft having one or more gas turbine engines,comprising:

-   -   measuring a swell parameter of a seal material using the method        of the ninth aspect; and    -   operating the aircraft according to the swell parameter.

Operating the aircraft according to the swell parameter may includeproviding the one or more gas turbine engines with fuel having adifferent characteristic compared to the fuel for which the seal swellhas been determined. This may be done by either refuelling the aircraftor by talking fuel from a different source aboard the aircraft.

Providing fuel having a different characteristic may comprise any one ormore of: i) providing fuel with a relatively higher aromatic content;ii) providing fuel with a relatively lower SAF content; and/or iii)providing kerosene.

According to an eleventh aspect, the present application provides anaircraft comprising the fuel determination system of the sixth aspect,the aircraft further comprising a control system arranged to controloperation of the aircraft according to the one or more fuelcharacteristics determined by the fuel determination system.

According to a twelfth aspect, the present application provides anaircraft according to the eight aspect, the aircraft further comprisinga control system arranged to control operation of the aircraft accordingto the swell parameter measured by the detection device.

According to a thirteenth aspect, the present application provides amethod of determining one or more fuel characteristics of an aviationfuel for powering a gas turbine engine of an aircraft, the methodcomprising:

-   -   measuring one or more trace substance parameters of the fuel,        the one or more trace substance parameters each associated with        a respective trace substance in the fuel; and    -   determining one or more fuel characteristics of the fuel based        on the one or more trace substance parameters.

The inventors have determined that by measuring trace substanceparameters of a fuel, certain characteristics of that fuel may bedetermined. Such trace substances may be present in only a trace amountand may indicate an associated characteristic of the fuel by theirpresence, concentration or absence in the fuel.

At least one of the one or more trace substance parameters may indicatethe presence or a concentration of the associated trace substance in thefuel.

At least one of the one or more trace substance parameters may indicatethe absence of the associated trace substance in the fuel.

The one or more fuel characteristics determined may include:

-   -   a) a percentage of sustainable aviation fuel in the fuel; or    -   b) an indication that the fuel is a fossil fuel, for example        fossil kerosene fuel.

The trace substance associated with at least one of the substanceparameters may occur inherently in the fuel. For example, it may occurnaturally in the fuel as a result of its manufacture.

The trace substance parameters may include:

-   -   i) a concentration or amount of sulphur within the fuel; and/or    -   ii) a concentration or amount of aromatic hydrocarbon within the        fuel.

The trace substance associated with at least one of the trace substanceparameters may be added to the fuel to act as a tracer for detection.For example, the trace substance may be added to the fuel for thepurpose of acting as a tracer to indicate a certain characteristic orcharacteristics of the fuel.

Measuring the one or more trace substance parameters may compriseperforming spectroscopy on the fuel. Preforming spectroscopy on the fuelmay comprise performing Fourier Transform Infrared (FT-IR) orUltraviolet Visual (UV-Vis) spectroscopy.

Measuring the one or more trace substance parameters may compriseperforming fluorescence detection.

Measuring the one or more trace substance parameters may compriseperforming a measurement on fuel onboard the aircraft.

Measuring the one or more trace substance parameters may compriseperforming a measurement on fuel within, sampled from, or beingdelivered to a fuel tank of the aircraft.

Measuring the one or more trace substance parameters may compriseperforming a measurement on fuel during use of the gas turbine engine.This may include performing a measurement on fuel while it is within afuel system of the gas turbine engine.

Measuring the one or more trace substance parameters may compriseperforming a measurement on fuel outside of the aircraft during a fuelloading process in which the fuel tank or tanks of the aircraft areloaded with fuel. A sensor to measure the trace substance parameters maytherefore be located in a fuel loading system for the aircraft.

According to a fourteenth aspect, the present application provides afuel characteristic determination system for determining one or morefuel characteristics of an aviation fuel for powering a gas turbineengine of an aircraft, the system comprising:

-   -   a sensor configured to measure one or more trace substance        parameters of the fuel, the one or more trace substance        parameters each associated with a respective trace substance in        the fuel; and    -   and determination module configured to determine one or more        fuel characteristics of the fuel based on the one or more trace        substance parameters.

At least one of the one or more trace substance parameters may indicatethe presence or a concentration of the associated trace substance in thefuel.

At least one of the one or more trace substance parameters may indicatethe absence of the associated trace substance in the fuel.

The one or more fuel characteristics determined may include:

-   -   a) a percentage of sustainable aviation fuel in the fuel; or    -   b) an indication that the fuel is a fossil fuel, for example,        fossil kerosene fuel.

The trace substance associated with at least one of the trace substanceparameters may occur inherently in the fuel.

The trace substance parameter may be:

-   -   a) a concentration or amount of sulphur within the fuel (F);        and/or    -   b) a concentration or amount of aromatic hydrocarbon in the fuel        (F).

The trace substance associated with at least one of the trace substanceparameters may be added to the fuel to act as a tracer for detection.

The sensor may comprise a spectroscopy device. The spectroscopy devicemay preferably be Fourier Transform Infrared (FT-IR) or UltravioletVisual (UV-Vis) spectroscopy device.

The sensor may comprise a fluorescence detection device.

The sensor may be configured to perform a measurement on fuel onboardthe aircraft.

The sensor may be arranged to perform a measurement on fuel within,sampled from, or being delivered to a fuel tank of the aircraft.

The sensor may be arranged to perform a measurement on fuel during useof the gas turbine engine.

The sensor may be located within a fuel system of the gas turbineengine.

The sensor nay be arranged to perform a measurement on fuel outside ofthe aircraft that is being loaded onto the aircraft during a fuelloading process. For example, the sensor may be located withinre-fuelling system of the aircraft.

According to a fifteenth aspect, the present application provides amethod of operating an aircraft having a gas turbine engine, the methodcomprising:

-   -   determining one or more fuel characteristics using the method of        the thirteenth aspect; and operating the aircraft according to        the one or more fuel characteristics.

Operating the aircraft according to the one or more fuel characteristicsmay comprise:

-   -   a) modifying a control parameter of the aircraft, preferably a        control parameter of the gas turbine engine, in response to the        one or more fuel characteristics; and/or    -   b) providing a fuel having different fuel characteristics during        refuelling of the aircraft.

According to a sixteenth aspect, the present application provides anaircraft comprising the fuel characteristic determination system of thefourteenth aspect, the aircraft further comprising a control systemarranged to control operation of the aircraft according to the one ormore fuel characteristics determined by the fuel characteristicdetermination system.

According to a seventeenth aspect, there is provide a method ofdetermining one or more fuel characteristics of an aviation fuelsuitable for powering a gas turbine engine of an aircraft, the methodcomprising:

-   -   passing UV-visual spectrum light through the fuel;    -   measuring a transmittance parameter indicating the transmittance        of light through the fuel;    -   determining one or more fuel characteristics of the fuel based        on the transmittance parameter; and    -   communicating the one or more fuel characteristic to a control        system of the gas turbine engine or the aircraft.

The inventors have determined that by measuring the UV-Visual spectrumlight transmittance properties of an aviation fuel, characteristics ofthat fuel can be determined and communicated to a control module of theaircraft so that the aircraft can be operated based on them.

The transmittance parameter may indicate the transmittance of theUV-visual spectrum light as a function of wavelength.

The one or more fuel characteristics determined based on thetransmittance parameter may include a hydrocarbon distribution of thefuel.

The one or more fuel characteristics may include any one or more of: (i)an aromatic hydrocarbon content of the fuel; ii) a percentage ofsustainable aviation fuel in the fuel; and/or iii) an indication as towhether the fuel is fossil fuel, e.g. fossil kerosene.

The UV-visual spectrum light may be passed though the fuel within,sampled from, or being delivered to a fuel tank of the aircraft.

The UV-visual spectrum light may be passed though the fuel during use ofthe gas turbine engine. The UV-visual spectrum light may be passedthrough the fuel within a fuel system of the gas turbine engine.

The UV-visual spectrum light may be passed though the fuel outside ofthe aircraft during a fuel loading process in which the fuel tank ortanks of the aircraft are loaded with fuel. A UV-vis transmittancesensor may therefore be provided within a re-fuelling system of theaircraft.

According to an eighteenth aspect, the present application provides afuel characteristic determination system for determining one or morefuel characteristics of an aviation fuel suitable for powering a gasturbine engine of an aircraft, the system comprising:

-   -   a UV-Vis sensor comprising a UV-visual spectrum light source        arranged to pass UV-visual spectrum light through the fuel, the        UV-Vis sensor further comprising a transmittance detector        arranged to measure a transmittance parameter indicative of the        transmittance of the UV-visual spectrum light through the fuel;        and    -   a determination module arranged to determine one or more fuel        characteristics of the fuel based on the transmittance        parameter, wherein the determination module is arranged to        communicate the one or more fuel characteristics to a control        module of the gas turbine engine or aircraft.

The transmittance parameter may indicate the transmittance of theUV-visual spectrum light as a function of wavelength.

The one or more fuel characteristics determined based on thetransmittance parameter may include a hydrocarbon distribution of thefuel.

The one or more fuel characteristics may include any one or more of:

-   -   (i) an aromatic hydrocarbon content of the fuel (F);    -   ii) a percentage of sustainable aviation fuel in the fuel (F);        and/or    -   iii) an indication as to whether the fuel is fossil fuel, e.g.        fossil kerosene.

The light source may be arranged to pass light though fuel which iswithin, sampled from, or being delivered to a fuel tank of the aircraft.

The light source may be arranged to pass light though fuel during use ofthe gas turbine engine. The light source may be arranged to pass lightthrough fuel while it is within a fuel system of the gas turbine engine.

The light source may be arranged to pass light though fuel which isoutside of the aircraft during a fuel loading process in which the fueltank or tanks of the aircraft are loaded with fuel. The light source maytherefore be provided in a re-fuelling system of the aircraft.

According to a nineteenth aspect, the present application provides amethod of operating an aircraft having a gas turbine engine, the methodcomprising:

-   -   determining one or more fuel characteristics using the method of        the seventeenth aspect; and    -   operating the aircraft according to the one or more fuel        characteristics.

Operating the aircraft according to the one or more fuel characteristicsmay comprise:

-   -   a) modifying a control parameter of the aircraft, preferably a        control parameter of the gas turbine engine, in response to the        one or more fuel characteristics; and/or    -   b) providing a fuel having different fuel characteristics during        refuelling of the aircraft.

According to a twentieth aspect, the present application provides anaircraft comprising the fuel determination system of the eighteenthaspect, the aircraft further comprising a control system arranged tocontrol operation of the aircraft according to the one or more fuelcharacteristics determined by the fuel characteristic determinationsystem.

According to a twenty first aspect, there is provided a method ofdetermining one or more fuel characteristic of an aviation fuel suitablefor powering a gas turbine engine of an aircraft, the method comprising:

-   -   determining, during use of the gas turbine engine, one or more        contrail parameters related to contrail formation by the gas        turbine engine, wherein determining the one or more contrail        parameters comprises performing a sensor measurement on a region        behind the gas turbine engine in which a contrail is or can be        formed; and    -   determining one or more fuel characteristics of the fuel based        on the one or more contrail parameters.

The inventors have determined that by performing a sensor measurementsensitive to the formation of contrails on the exhaust plume of a gasturbine engine the characteristics of the fuel being burnt by the gasturbine engine can be determined.

The one or more contrail parameters may include a parameter indicativeof the degree of contrail formation taking place. The one or morecontrol parameters may include a parameter indicative of the presence orabsence of a contrail produced by the gas turbine engine.

Determining the one or more contrail parameters may comprise measuringelectromagnetic radiation reflected and/or re-emitted by a contrail.

Determining the one or more contrail parameters may comprise detectingthe presence or absence of a contrail, or the degree to which a contrailis formed, in an image of the region behind the gas turbine engine.

The one or more fuel characteristics may be further determined based onone or more ambient atmospheric condition parameters, each indicative ofthe ambient atmospheric conditions in which the gas turbine engine iscurrently operating.

The method may further comprise obtaining the one or more ambientatmospheric conditions from a source of meteorological data providingreal-time or expected information on the ambient atmospheric conditions.

The method may further comprise obtaining the one or more ambientatmospheric conditions from a sensor arranged to measure the ambientconditions in the vicinity of the aircraft.

The one or more fuel characteristics may be further determined based onone or more engine or aircraft operating parameters.

The one or more fuel characteristics may be determined based onmeasuring the value of a varying parameter at which contrail formationbegins. The varying parameter may be a varying engine operationparameter and/or a varying ambient condition parameter.

The one or more fuel characteristics determined may include any one ormore of: (i) a hydrocarbon distribution of the fuel; (ii) a percentageof sustainable aviation fuel in the fuel; (iii) an aromatic hydrocarboncontent of the fuel; and/or (iv) an indication that the fuel is a fossilfuel e.g. kerosene.

According to a twenty second aspect, there is provided a fuelcharacteristic determination system for determining one or more fuelcharacteristic of an aviation fuel suitable for powering a gas turbineengine of an aircraft, the system comprising:

-   -   a contrail sensor arranged to determine one or more contrail        parameters related to contrail formation by the gas turbine        engine, the contrail sensor being arranged to perform a sensor        measurement on a region behind the gas turbine engine in which a        contrail is or can be formed; and    -   a fuel characteristic determination module arranged to determine        one or more fuel characteristics of the fuel based on the one or        more contrail parameters.

The one or more contrail parameters may include a parameter indicativeof the degree of contrail formation taking place. The one or morecontrol parameters may preferably indicate a presence or absence of acontrail produced by the gas turbine engine.

The contrail sensor may be arranged to measure electromagnetic radiationreflected and/or re-emitted by a contrail.

The contrail sensor may be arranged to detect the presence or absence ofa contrail, or the degree to which a contrail is formed, in an image ofthe region behind the gas turbine engine.

The fuel characteristic determination module may be further arranged todetermine the one or more fuel characteristics based on one or moreambient atmospheric condition parameters, each indicative of the ambientatmospheric conditions in which the gas turbine engine is currentlyoperating.

The fuel characteristics determination module may be further arranged toobtain the one or more ambient atmospheric conditions from a source ofmeteorological data providing real-time or expected information on theambient atmospheric conditions.

The fuel characteristics determination module may be further arranged toobtain the one or more ambient atmospheric conditions from a sensorarranged to measure the ambient conditions in the vicinity of theaircraft.

The fuel characteristic module may be further arranged to determine theone or more fuel characteristics based on one or more engine or aircraftoperating parameters. The one or more aircraft and engine parameters mayinclude a temperature of fuel entering a combustor of the gas turbineengine.

The one or more fuel characteristics may be determined based onmeasuring the value of a varying engine or aircraft operating parameterat which contrail formation begins.

The one or more fuel characteristics determined may include any one ormore of: (i) a hydrocarbon distribution of the fuel; (ii) a percentageof sustainable aviation fuel in the fuel; (iii) an aromatic hydrocarboncontent of the fuel; and/or (iv) an indication that the fuel is a fossilfuel e.g. kerosene.

According to a twenty third aspect, the present application provides amethod of operating an aircraft having a gas turbine engine, the methodcomprising:

-   -   determining one or more fuel characteristics using the method of        the twenty first aspect; and    -   operating the gas turbine engine or aircraft according to the        one or more fuel characteristics.

Operating the aircraft according to the one or more fuel characteristicsmay comprise any one or more of:

-   -   i) changing an operating parameter of a heat management system        of the gas turbine engine;    -   ii) changing a fuel temperature of the fuel within the gas        turbine engine; and/or    -   iii) changing a flight characteristic of the aircraft,        preferably an altitude of the aircraft, further preferably a        cruise altitude.

According to a twenty fourth aspect, the present application provides amethod of operating an aircraft having a gas turbine engine, the methodcomprising:

-   -   determining, during use of the gas turbine engine, one or more        contrail parameters related to contrail formation by the gas        turbine engine, wherein determining the one or more contrail        parameters comprises performing a sensor measurement on a region        behind the gas turbine engine in which a contrail is or can be        formed, and wherein the one or more control parameters are        determined during a varying operation of the aircraft and        correspond to a value of a varying parameter at which a control        begins to form; and    -   controlling the operating parameter of the aircraft or gas        turbine engine according to the one or more contrail parameters.

The varying operation of the aircraft may be a climb phase of operationof the aircraft

According to a twenty fifth aspect, the present application provides anaircraft comprising the fuel determination system of the twenty secondaspect, the aircraft further comprising a control system arranged tocontrol operation of the aircraft according to the one or more fuelcharacteristics determined by the fuel determination system.

According to a twenty sixth aspect, the present application provides amethod of determining one or more fuel characteristics of an aviationfuel suitable for powering a gas turbine engine of an aircraft, themethod comprising:

-   -   determining, during use of the gas turbine engine, one or more        exhaust content parameters by performing a sensor measurement on        an exhaust of the gas turbine engine; and    -   determining one or more fuel characteristics of the fuel based        on the one or more exhaust parameters.

The inventors have determined that characteristics of a fuel being usedby a gas turbine engine can be determined by performing measurement onthe exhaust gases produced by the engine while it is in use.

The one or more exhaust content parameters may include a parameterindicative of the nvPM content of the exhaust.

Performing the sensor measurement may comprise performing a laserinduced incandescence measurement to determine the volume concentrationof nvPM in the exhaust.

Performing the sensor measurement may comprise performing a condensationparticle count measurement to determine an nvPM number in the exhaust.

The one or more exhaust content parameters may include a parameterindicative of the SO₂, CO₂ or CO content of the exhaust.

Performing the sensor measurement may comprise performing anon-dispersive Infrared absorption measurement.

The one or more exhaust content parameters may include a sulphateaerosol content of the exhaust. Performing the sensor measurement maycomprise performing an aerosol mass spectrometer measurement todetermine the presence of sulphates within the exhaust.

The one or more fuel characteristics may be further determined based onone or more ambient atmospheric condition parameters, each indicative ofthe ambient atmospheric conditions in which the gas turbine engine iscurrently operating.

The one or more fuel characteristics may be further determined based onone or more engine operating parameters. The operating parameters mayinclude an engine power setting.

The one more fuel characteristics may be determined based on an exhaustcontent parameter measured at a first engine operation condition inwhich emission of the respective substance being measured is greaterthan at a second engine operation condition.

The one or more fuel characteristics may be determined based on acomparison of exhaust content parameters determined at different engineoperation conditions.

The one or more fuel characteristics may include any one of more of: (i)a hydrogen to carbon ratio of the fuel; (ii) a percentage of sustainableaviation fuel in the fuel; (iii) an aromatic hydrocarbon content of thefuel; (iv) a naphthalene content of the fuel; and/or (v) a sulphurcontent of the fuel.

According to the twenty seventh aspect, the present application providesa fuel characteristic determination system for determining one or morefuel characteristic of an aviation fuel suitable for powering a gasturbine engine of an aircraft, the system comprising:

-   -   an exhaust sensor arranged to determine one or more exhaust        content parameters, the exhaust sensor being arranged to perform        a measurement on an exhaust of the gas turbine engine; and    -   a fuel characteristic determination module arranged to determine        one or more fuel characteristics of the fuel based on the one or        more exhaust content parameters.

The exhaust sensor may be arranged to determine one or more exhaustcontent parameters that include a parameter indicative of the nvPMcontent of the exhaust.

The exhaust sensor may comprise a laser induced incandescencemeasurement device arranged to determine the volume concentration ofnvPM in the exhaust.

The exhaust sensor may comprise a condensation particle count devicearranged to determine an nvPM number in the exhaust.

The exhaust sensor may be arranged to determine one or more exhaustcontent parameters that include a parameter indicative of the SO₂, CO₂or CO content of the exhaust. The exhaust sensor may comprise anon-dispersive Infrared absorption measurement device.

The exhaust sensor may be arranged to determine one or more exhaustcontent parameters that include a sulphate aerosol content of theexhaust. The exhaust sensor may comprise an aerosol mass spectrometermeasurement device arranged to measure a sulphate mass in the exhaust.

The one or more fuel characteristics may be further determined by thefuel characteristic determination module based on:

-   -   i) one or more ambient atmospheric condition parameters, each        indicative of the ambient atmospheric conditions in which the        gas turbine engine is currently operating; and/or    -   ii) one or more engine operating parameters, preferably        including an engine power setting.

The one or more fuel characteristics may include any one of more of: (i)a hydrogen to carbon ratio of the fuel; (ii) a percentage of sustainableaviation fuel in the fuel; (iii) an aromatic hydrocarbon content of thefuel; (iv) a naphthalene content of the fuel; and/or (v) a sulphurcontent of the fuel.

According to a twenty eighth aspect, the present application provides amethod of operating an aircraft having a gas turbine engine, the methodcomprising:

-   -   determining one or more fuel characteristics using the method of        the twenty sixth aspect; and    -   operating the aircraft according to the one or more fuel        characteristics.

According to a twenty ninth aspect, the present application provides anaircraft comprising a gas turbine engine and the fuel characteristicsdetermination system of the twenty seventh aspect, the aircraft furthercomprising a control system arranged to control operation of theaircraft according to the one or more fuel characteristics determined bythe fuel characteristic determination system.

According to a thirtieth aspect, the present application provides amethod of determining one or more fuel characteristics of an aviationfuel used for powering a gas turbine engine of an aircraft, the methodcomprising:

-   -   determining one or more performance parameters of the gas        turbine engine during a first time period of operation of the        gas turbine engine;    -   determining one or more fuel characteristics of the fuel based        on the one or more performance parameters.

The inventors have determined that the characteristics of a fuel beingused by a gas turbine engine can be determined during use of that enginebased on an observation of performance parameters of the engine. Bydetermining one or more performance parameters of the engine,characteristics of the fuel can be determined based on those performanceparameters during a different, later period of the engine operation.

The fuel characteristics may be determined during a second later periodof operation.

The first period of operation may be a first flight phase, and thesecond period of operation may be a second flight phase, different fromthe first.

The first flight phase may be a take-off and/or climb phase, and thesecond phase flight phase may be a cruise phase or a descent phase.

The one or more performance parameters may include any one or more of:

-   -   a) a rotation speed of a fan of the gas turbine engine;    -   b) a turbine entry temperature of the gas turbine engine; and/or    -   c) a combustor fuel to air ratio of the gas turbine engine (this        may be defined as the ratio of mass of fuel flow to the        combustor compared to the core air flow).

Determining the one or more fuel characteristics may comprise comparingeach of the one or more determined performance parameters with areference performance parameter corresponding to operation of the gasturbine engine with a fuel having a known fuel characteristic.

Determining the one or more performance parameters may comprisedetermining a plurality of different performance parameters. The one ormore fuel characteristics may be determined based on the plurality ofperformance parameters. The plurality of performance parameters mayinclude at least two different performance parameters, and preferably atleast three different performance parameters.

The one or more fuel characteristics determined may include any one ormore of: (i) a hydrocarbon distribution of the fuel; (ii) a percentageof sustainable aviation fuel in the fuel; (iii) an aromatic hydrocarboncontent of the fuel; and/or (iv) an indication that the fuel is a fossilfuel e.g. kerosene.

According to a thirty first aspect, there is provided a method ofoperating an aircraft having a gas turbine engine, the methodcomprising:

-   -   determining one or more fuel characteristics using the method of        the thirtieth aspect;    -   operating the aircraft according to the one or more fuel        characteristics during a or the later the second period of        operation of the gas turbine engine.

The aircraft may comprise a plurality of fuel tanks. The aircraft may beonly operated according to the one or more fuel characteristics duringthe second period of operation if fuel is being used from the same fueltank, or fuel known to have the same fuel characteristics, as during thefirst period of operation.

Operating the aircraft according to the one or more fuel characteristicsmay comprises modifying a control parameter of the aircraft, preferablya control parameter of the gas turbine engine, in response to the one ormore fuel characteristics.

Operating the aircraft according to the one or more fuel characteristicsmay comprise any one or more of: i) changing fuel burn parameter of thegas turbine engine; ii) changing an operating parameter of heatmanagement system of the gas turbine engine; iii) adjusting a fueltemperature of the fuel within the gas turbine engine; iv) adjusting aflight characteristic of the aircraft, preferably an altitude of theaircraft, further preferably a cruise altitude.

According to a thirty second aspect, the present application provides afuel characteristic determination system for determining one or morefuel characteristics of an aviation fuel for powering a gas turbineengine of an aircraft, the system comprising:

-   -   a performance parameter sensor configured to determine one or        more performance parameters of the gas turbine engine during a        first time period of operation of the gas turbine engine; and    -   a fuel characteristic determination module configured to        determine one or more fuel characteristics of the fuel based on        the one or more performance parameters.

The fuel characteristic determine module may be configured to determinethe one or more fuel characteristics during a second later period ofoperation.

The first period of operation may be a first flight phase, and thesecond period of operation may be a second flight phase, different fromthe first.

The first flight phase may be a take-off and/or climb phase, and thesecond phase flight phase may be a cruise phase or a descent phase.

The one or more performance parameters may include any one or more of:

-   -   a) a rotation speed of a fan of the gas turbine engine;    -   b) a turbine entry temperature of the gas turbine engine; and/or    -   c) a combustor fuel to air ratio of the gas turbine engine (this        may be defined as the ratio of mass of fuel flow to the        combustor compared to the core air flow).

The fuel characteristic determination module may be arranged todetermine the one or more fuel characteristics by comparing each of theone or more determined performance parameters with a referenceperformance parameter corresponding to operation of the gas turbineengine with a fuel having a known fuel characteristic.

The fuel characteristic determination module may be arranged todetermine the one or more fuel characteristics by obtaining a pluralityof different performance parameters from the sensor or additionalsensors. The one or more fuel characteristics may be determined based onthe plurality of performance parameters. The plurality of performanceparameters may include at least two different performance parameters,and preferably at least three different performance parameters.

The one or more fuel characteristics determined may include any one ormore of (i) a hydrocarbon distribution of the fuel; (ii) a percentage ofsustainable aviation fuel in the fuel; (iii) an aromatic hydrocarboncontent of the fuel; and/or (iv) an indication that the fuel is a fossilfuel e.g. kerosene.

According to a thirty third aspect, the present application provides anaircraft comprising the fuel characteristic determination system of thethirty second aspect, the aircraft further comprising a control systemarranged to control operation of the aircraft according to the one ormore fuel characteristics determined by the fuel determination system.

According to a thirty fourth aspect, the present application provides amethod of generating a maintenance schedule for an aircraft having oneor more gas turbine engines powered by an aviation fuel, comprising:

-   -   determining one or more fuel characteristics of the fuel; and    -   generating a maintenance schedule according to the one or more        fuel characteristics.

The inventors have determined that the characteristics of the fuel thathas been used to power the gas turbine have an effect on the operationof the gas turbine engine and the aircraft in general and so may requirea change in a maintenance schedule for that aircraft. A maintenanceschedule for the aircraft can therefore be advantageously generatedbased on the characteristics of the fuel with which it has beenoperated. This may allow the fuel that has actually been used to powerthe aircraft to be taken into account when performing maintenance.

Generating the maintenance schedule may comprise modifying an existingmaintenance schedule for the aircraft according to the determined one ormore fuel characteristics.

Generating the maintenance schedule may comprise comparing the one ormore determined fuel characteristics to an expected fuel characteristic.

The existing maintenance schedule may be associated with the expectedfuel characteristic. Modifying the existing maintenance schedule may bedone in response to determining a deviation from the expected fuelcharacteristic.

Determining the one or more fuel characteristics may comprise makingperiodic determinations of a fuel characteristic or characteristics.

The one or more fuel characteristics may include any one or more of (i)a hydrocarbon distribution of the fuel; (ii) a percentage of sustainableaviation fuel in the fuel; (iii) an aromatic hydrocarbon content of thefuel; and/or (iv) an indication that the fuel is a fossil fuel e.g.kerosene.

Determining the one or more fuel characteristics may comprise measuringa change in the properties of a sensor component exposed to the fuelused to power the one or more gas turbine engines.

The one or more fuel characteristics may indicate a threshold level offuel coking or surface deposit formation has occurred.

The sensor component may be a piezoelectric crystal. Determining the oneor more fuel characteristics may comprise measuring a vibrationparameter of the piezoelectric crystal.

The sensor component may comprise a seal material and the one or morefuel characteristics may indicate whether a threshold level of swell ofthe seal material exposed to the fuel has occurred. The seal materialmay be the same as at least one seal provided in a fuel system of theone or more gas turbine engines. The seal material may be a nitrile sealmaterial.

According to a thirty fifth aspect, the present application provides amethod of maintaining an aircraft, comprising:

-   -   generating a maintenance schedule using the method of the thirty        fourth aspect; and    -   performing maintenance on the aircraft according to the        generated maintenance schedule.

According to a thirty sixth aspect, the present application provides amaintenance schedule generation system for generating a maintenanceschedule for an aircraft having one or more gas turbine engines,comprising:

-   -   a fuel characteristic determination module configured to        determine one or more fuel characteristics of a fuel provided to        the one or more gas turbine engines of the aircraft; and    -   a maintenance schedule generation module configured to generate        a maintenance schedule according to the one or more fuel        characteristics.

The generation module may be configured to modify an existingmaintenance schedule for the aircraft according to the determined one ormore fuel characteristics.

The maintenance schedule generation module may be configured to comparethe one or more determined fuel characteristics to an expected fuelcharacteristic.

The existing maintenance schedule may be associated with the expectedfuel characteristic. The maintenance schedule generation module may beconfigured to modify the existing maintenance schedule in response todetermining a deviation from the expected fuel characteristic.

The fuel characteristic determination module may be configured toperform periodic determinations of the one or more fuel characteristics.

The one or more fuel characteristics may include any one or more of (i)a hydrocarbon distribution of the fuel; (ii) a percentage of sustainableaviation fuel in the fuel; (iii) an aromatic hydrocarbon content of thefuel; and/or (iv) an indication that the fuel is a fossil fuel e.g.kerosene.

The fuel characteristic determination module may be arranged to receivea sensor parameter indicative of a measured change in the properties ofa sensor component exposed to the fuel used to power the one or more gasturbine engines, and base the determination of the one or more fuelcharacteristics on the received sensor parameter.

The one or more fuel characteristics may indicate a threshold level offuel coking or surface deposit formation has occurred. The sensorcomponent may be a piezoelectric crystal, and determining the one ormore fuel characteristics may comprises measuring a vibration parameterof the piezoelectric crystal.

The sensor component may comprise a seal material. The one or more fuelcharacteristics may indicate whether a threshold level of swell of theseal material exposed to the fuel has occurred. The seal material may bethe same as at least one seal provided in a fuel system of the one ormore gas turbine engines. The seal material may be a nitrile sealmaterial.

According to a thirty seventh aspect, the present application providesan aircraft having one or more gas turbine engines, the aircraftcomprising the maintenance schedule generation system of the thirtysixth aspect.

As used herein, the term “fuel characteristics” refers to inherent fuelproperties such as fuel composition, not variable properties such asvolume or temperature. Examples of fuel characteristics of a fuelinclude:

-   -   (i) the percentage of sustainable aviation fuel in the fuel;    -   (ii) the aromatic hydrocarbon content of the fuel;    -   (iii) the multi-aromatic hydrocarbon content of the fuel;    -   (iv) the percentage of nitrogen-containing species in the fuel;    -   (v) the presence or percentage of a trace species or trace        element in the fuel (e.g. a trace substance inherently present        in the fuel, or one added deliberately to act as a tracer);    -   (vi) the hydrogen to carbon ratio of the fuel;    -   (vii) the hydrocarbon distribution of the fuel;    -   (viii) the level of non-volatile particulate matter (nvPM)        emissions on combustion (e.g. on combustion for a given        combustor design, at a given operating condition (FAR, T30,        combustor mode etc));    -   (ix) the naphthalene content of the fuel;    -   (x) the sulphur content of the fuel;    -   (xi) the cycloparaffin content of the fuel;    -   (xii) the oxygen content of the fuel;    -   (xiii) the thermal stability of the fuel (e.g. thermal breakdown        temperature);    -   (xiv) the level of coking of the fuel;    -   (xv) an indication that the fuel is a fossil fuel, for example        fossil kerosene; and    -   (xvi) one or more properties such as density, viscosity,        calorific value, and/or heat capacity.

In any aspect or statement above involving operating an aircraftaccording to one or more determined fuel characteristics, operating theaircraft may comprise modifying a control parameter of the aircraft, andspecifically a control parameter of the gas turbine engine, in responseto the one or more fuel characteristics as described anywhere herein.Operating the aircraft may additionally or alternatively compriseproviding fuel having a different fuel characteristics, e.g. duringrefuelling as described elsewhere herein.

As noted elsewhere herein, the present disclosure may relate to a gasturbine engine. Such a gas turbine engine may comprise an engine corecomprising a turbine, a combustor, a compressor, and a core shaftconnecting the turbine to the compressor. Such a gas turbine engine maycomprise a fan (having fan blades) located upstream of the engine core.Alternatively, in some examples, the gas turbine engine may comprise afan located downstream of the engine core. Thus, the gas turbine enginemay be an open rotor or a turboprop engine.

Where the gas turbine engine is an open rotor or a turboprop engine, thegas turbine engine may comprise two contra-rotating propeller stagesattached to and driven by a free power turbine via a shaft. Thepropellers may rotate in opposite senses so that one rotates clockwiseand the other anti-clockwise around the engine's rotational axis.Alternatively, the gas turbine engine may comprise a propeller stage anda guide vane stage configured downstream of the propeller stage. Theguide vane stage may be of variable pitch. Accordingly, high-pressure,intermediate pressure, and free power turbines respectively may drivehigh and intermediate pressure compressors and propellers by suitableinterconnecting shafts. Thus, the propellers may provide the majority ofthe propulsive thrust.

Where the gas turbine engine is an open rotor or a turboprop engine, oneor more of the propellor stages may be driven by a gearbox of the typedescribed.

Arrangements of the present disclosure may be particularly, although notexclusively, beneficial for fans that are driven via a gearbox.Accordingly, the gas turbine engine may comprise a gearbox that receivesan input from the core shaft and outputs drive to the fan so as to drivethe fan at a lower rotational speed than the core shaft. The input tothe gearbox may be directly from the core shaft, or indirectly from thecore shaft, for example via a spur shaft and/or gear. The core shaft mayrigidly connect the turbine and the compressor, such that the turbineand compressor rotate at the same speed (with the fan rotating at alower speed).

The gas turbine engine as described and/or claimed herein may have anysuitable general architecture. For example, the gas turbine engine mayhave any desired number of shafts that connect turbines and compressors,for example one, two or three shafts. Purely by way of example, theturbine connected to the core shaft may be a first turbine, thecompressor connected to the core shaft may be a first compressor, andthe core shaft may be a first core shaft. The engine core may furthercomprise a second turbine, a second compressor, and a second core shaftconnecting the second turbine to the second compressor. The secondturbine, second compressor, and second core shaft may be arranged torotate at a higher rotational speed than the first core shaft.

In such an arrangement, the second compressor may be positioned axiallydownstream of the first compressor. The second compressor may bearranged to receive (for example directly receive, for example via agenerally annular duct) flow from the first compressor.

The gearbox may be arranged to be driven by the core shaft that isconfigured to rotate (for example in use) at the lowest rotational speed(for example the first core shaft in the example above). For example,the gearbox may be arranged to be driven only by the core shaft that isconfigured to rotate (for example in use) at the lowest rotational speed(for example only be the first core shaft, and not the second coreshaft, in the example above). Alternatively, the gearbox may be arrangedto be driven by any one or more shafts, for example the first and/orsecond shafts in the example above.

The gearbox may be a reduction gearbox (in that the output to the fan isa lower rotational rate than the input from the core shaft). Any type ofgearbox may be used. For example, the gearbox may be a “planetary” or“star” gearbox, as described in more detail elsewhere herein. Thegearbox may have any desired reduction ratio (defined as the rotationalspeed of the input shaft divided by the rotational speed of the outputshaft), for example greater than 2.5, for example in the range of from 3to 4.2, or 3.2 to 3.8, for example on the order of or at least 3, 3.1,3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4, 4.1 or 4.2. The gear ratiomay be, for example, between any two of the values in the previoussentence. Purely by way of example, the gearbox may be a “star” gearboxhaving a ratio in the range of from 3.1 or 3.2 to 3.8. In somearrangements, the gear ratio may be outside these ranges.

In any gas turbine engine as described and/or claimed herein, fuel of agiven composition or blend is provided to a combustor, which may beprovided axially downstream of the fan and compressor(s). For example,the combustor may be directly downstream of (for example at the exit of)the second compressor, where a second compressor is provided. By way offurther example, the flow at the exit to the combustor may be providedto the inlet of the second turbine, where a second turbine is provided.The combustor may be provided upstream of the turbine(s).

The or each compressor (for example the first compressor and secondcompressor as described above) may comprise any number of stages, forexample multiple stages. Each stage may comprise a row of rotor bladesand a row of stator vanes, which may be variable stator vanes (in thattheir angle of incidence may be variable). The row of rotor blades andthe row of stator vanes may be axially offset from each other.

The or each turbine (for example the first turbine and second turbine asdescribed above) may comprise any number of stages, for example multiplestages. Each stage may comprise a row of rotor blades and a row ofstator vanes. The row of rotor blades and the row of stator vanes may beaxially offset from each other.

Each fan blade may be defined as having a radial span extending from aroot (or hub) at a radially inner gas-washed location, or 0% spanposition, to a tip at a 100% span position. The ratio of the radius ofthe fan blade at the hub to the radius of the fan blade at the tip maybe less than (or on the order of) any of: 0.4, 0.39, 0.38, 0.37, 0.36,0.35, 0.34, 0.33, 0.32, 0.31, 0.3, 0.29, 0.28, 0.27, 0.26, or 0.25. Theratio of the radius of the fan blade at the hub to the radius of the fanblade at the tip may be in an inclusive range bounded by any two of thevalues in the previous sentence (i.e. the values may form upper or lowerbounds), for example in the range of from 0.28 to 0.32. These ratios maycommonly be referred to as the hub-to-tip ratio. The radius at the huband the radius at the tip may both be measured at the leading edge (oraxially forwardmost) part of the blade. The hub-to-tip ratio refers, ofcourse, to the gas-washed portion of the fan blade, i.e. the portionradially outside any platform.

The radius of the fan may be measured between the engine centreline andthe tip of a fan blade at its leading edge. The fan diameter (which maysimply be twice the radius of the fan) may be greater than (or on theorder of) any of: 220 cm, 230 cm, 240 cm, 250 cm (around 100 inches),280 cm, 270 cm (around 105 inches), 280 cm (around 110 inches), 290 cm(around 115 inches), 300 cm (around 120 inches), 310 cm, 320 cm (around125 inches), 330 cm (around 130 inches), 340 cm (around 135 inches), 350cm, 360 cm (around 140 inches), 370 cm (around 145 inches), 380 (around150 inches) cm, 390 cm (around 155 inches), 400 cm, 410 cm (around 160inches) or 420 cm (around 165 inches). The fan diameter may be in aninclusive range bounded by any two of the values in the previoussentence (i.e. the values may form upper or lower bounds), for examplein the range of from 240 cm to 280 cm or 330 cm to 380 cm.

The rotational speed of the fan may vary in use. Generally, therotational speed is lower for fans with a higher diameter. Purely by wayof non-limitative example, the rotational speed of the fan at cruiseconditions may be less than 2500 rpm, for example less than 2300 rpm.Purely by way of further non-limitative example, the rotational speed ofthe fan at cruise conditions for an engine having a fan diameter in therange of from 220 cm to 300 cm (for example 240 cm to 280 cm or 250 cmto 270 cm) may be in the range of from 1700 rpm to 2500 rpm, for examplein the range of from 1800 rpm to 2300 rpm, for example in the range offrom 1900 rpm to 2100 rpm. Purely by way of further non-limitativeexample, the rotational speed of the fan at cruise conditions for anengine having a fan diameter in the range of from 330 cm to 380 cm maybe in the range of from 1200 rpm to 2000 rpm, for example in the rangeof from 1300 rpm to 1800 rpm, for example in the range of from 1400 rpmto 1800 rpm.

In use of the gas turbine engine, the fan (with associated fan blades)rotates about a rotational axis. This rotation results in the tip of thefan blade moving with a velocity U_(tip). The work done by the fanblades 13 on the flow results in an enthalpy rise dH of the flow. A fantip loading may be defined as dH/U_(tip) ², where dH is the enthalpyrise (for example the 1-D average enthalpy rise) across the fan andU_(tip) is the (translational) velocity of the fan tip, for example atthe leading edge of the tip (which may be defined as fan tip radius atleading edge multiplied by angular speed). The fan tip loading at cruiseconditions may be greater than (or on the order of) any of: 0.28, 0.29,0.30, 0.31, 0.32, 0.33, 0.34, 0.35, 0.36, 0.37, 0.38, 0.39 or 0.4 (allvalues being dimensionless). The fan tip loading may be in an inclusiverange bounded by any two of the values in the previous sentence (i.e.the values may form upper or lower bounds), for example in the range offrom 0.28 to 0.31, or 0.29 to 0.3.

Gas turbine engines in accordance with the present disclosure may haveany desired bypass ratio, where the bypass ratio is defined as the ratioof the mass flow rate of the flow through the bypass duct to the massflow rate of the flow through the core at cruise conditions. In somearrangements the bypass ratio may be greater than (or on the order of)any of the following: 10, 10.5, 11, 11.5, 12, 12.5, 13, 13.5, 14, 14.5,15, 15.5, 16, 16.5, 17, 17.5, 18, 18.5, 19, 19.5 or 20. The bypass ratiomay be in an inclusive range bounded by any two of the values in theprevious sentence (i.e. the values may form upper or lower bounds), forexample in the range of form 12 to 16, 13 to 15, or 13 to 14. The bypassduct may be substantially annular. The bypass duct may be radiallyoutside the core engine. The radially outer surface of the bypass ductmay be defined by a nacelle and/or a fan case.

The overall pressure ratio of a gas turbine engine as described and/orclaimed herein may be defined as the ratio of the stagnation pressureupstream of the fan to the stagnation pressure at the exit of thehighest pressure compressor (before entry into the combustor). By way ofnon-limitative example, the overall pressure ratio of a gas turbineengine as described and/or claimed herein at cruise may be greater than(or on the order of) any of the following: 35, 40, 45, 50, 55, 60, 65,70, 75. The overall pressure ratio may be in an inclusive range boundedby any two of the values in the previous sentence (i.e. the values mayform upper or lower bounds), for example in the range of from 50 to 70.

Specific thrust of an engine may be defined as the net thrust of theengine divided by the total mass flow through the engine. In someexamples, specific thrust may depend, for a given thrust condition, uponthe specific composition of fuel provided to the combustor. At cruiseconditions, the specific thrust of an engine described and/or claimedherein may be less than (or on the order of) any of the following: 110Nkg⁻¹s, 105 Nkg⁻¹s, 100 Nkg⁻¹s, 95 Nkg⁻¹s, 90 Nkg⁻¹s, 85 Nkg⁻¹s or 80Nkg⁻¹s. The specific thrust may be in an inclusive range bounded by anytwo of the values in the previous sentence (i.e. the values may formupper or lower bounds), for example in the range of from 80 Nkg⁻¹s to100 Nkg⁻¹s, or 85 Nkg⁻¹s to 95 Nkg⁻¹s. Such engines may be particularlyefficient in comparison with conventional gas turbine engines.

A gas turbine engine as described and/or claimed herein may have anydesired maximum thrust. Purely by way of non-limitative example, a gasturbine as described and/or claimed herein may be capable of producing amaximum thrust of at least (or on the order of) any of the following:160 kN, 170 kN, 180 kN, 190 kN, 200 kN, 250 kN, 300 kN, 350 kN, 400 kN,450 kN, 500 kN, or 550 kN. The maximum thrust may be in an inclusiverange bounded by any two of the values in the previous sentence (i.e.the values may form upper or lower bounds). Purely by way of example, agas turbine as described and/or claimed herein may be capable ofproducing a maximum thrust in the range of from 330 kN to 420 kN, forexample 350 kN to 400 kN. The thrust referred to above may be themaximum net thrust at standard atmospheric conditions at sea level plus15 degrees C. (ambient pressure 101.3 kPa, temperature 30 degrees C.),with the engine static.

In use, the temperature of the flow at the entry to the high pressureturbine may be particularly high. This temperature, which may bereferred to as TET, may be measured at the exit to the combustor, forexample immediately upstream of the first turbine vane, which itself maybe referred to as a nozzle guide vane. In some examples, TET may depend,for a given thrust condition, upon the specific composition of fuelprovided to the combustor. At cruise, the TET may be at least (or on theorder of) any of the following: 1400K, 1450K, 1500K, 1550K, 1600K or1650K. The TET at cruise may be in an inclusive range bounded by any twoof the values in the previous sentence (i.e. the values may form upperor lower bounds). The maximum TET in use of the engine may be, forexample, at least (or on the order of) any of the following: 1700K,1750K, 1800K, 1850K, 1900K, 1950K or 2000K. The maximum TET may be in aninclusive range bounded by any two of the values in the previoussentence (i.e. the values may form upper or lower bounds), for examplein the range of from 1800K to 1950K. The maximum TET may occur, forexample, at a high thrust condition, for example at a maximum take-off(MTO) condition.

A fan blade and/or aerofoil portion of a fan blade described and/orclaimed herein may be manufactured from any suitable material orcombination of materials. For example at least a part of the fan bladeand/or aerofoil may be manufactured at least in part from a composite,for example a metal matrix composite and/or an organic matrix composite,such as carbon fibre. By way of further example at least a part of thefan blade and/or aerofoil may be manufactured at least in part from ametal, such as a titanium based metal or an aluminium based material(such as an aluminium-lithium alloy) or a steel based material. The fanblade may comprise at least two regions manufactured using differentmaterials. For example, the fan blade may have a protective leadingedge, which may be manufactured using a material that is better able toresist impact (for example from birds, ice or other material) than therest of the blade. Such a leading edge may, for example, be manufacturedusing titanium or a titanium-based alloy. Thus, purely by way ofexample, the fan blade may have a carbon-fibre or aluminium based body(such as an aluminium lithium alloy) with a titanium leading edge.

A fan as described and/or claimed herein may comprise a central portion,from which the fan blades may extend, for example in a radial direction.The fan blades may be attached to the central portion in any desiredmanner. For example, each fan blade may comprise a fixture which mayengage a corresponding slot in the hub (or disc). Purely by way ofexample, such a fixture may be in the form of a dovetail that may slotinto and/or engage a corresponding slot in the hub/disc in order to fixthe fan blade to the hub/disc. By way of further example, the fan bladesmaybe formed integrally with a central portion. Such an arrangement maybe referred to as a bladed disc or a bladed ring. Any suitable methodmay be used to manufacture such a bladed disc or bladed ring. Forexample, at least a part of the fan blades may be machined from a blockand/or at least part of the fan blades may be attached to the hub/discby welding, such as linear friction welding.

The gas turbine engines described and/or claimed herein may or may notbe provided with a variable area nozzle (VAN). Such a variable areanozzle may allow the exit area of the bypass duct to be varied in use.The general principles of the present disclosure may apply to engineswith or without a VAN.

The fan of a gas turbine as described and/or claimed herein may have anydesired number of fan blades, for example 14, 16, 18, 20, 22, 24 or 26fan blades.

As used herein, the terms idle, taxi, take-off, climb, cruise, descent,approach, and landing have the conventional meaning and would be readilyunderstood by the skilled person. Thus, for a given gas turbine enginefor an aircraft, the skilled person would immediately recognise eachterm to refer to an operating phase of the engine within a given missionof an aircraft to which the gas turbine engine is designed to beattached.

In this regard, ground idle may refer to an operating phase of theengine where the aircraft is stationary and in contact with the ground,but where there is a requirement for the engine to be running. Duringidle, the engine may be producing between 3% and 9% of the availablethrust of the engine. In further examples, the engine may be producingbetween 5% and 8% of available thrust. In yet further examples, theengine may be producing between 6% and 7% of available thrust. Taxi mayrefer to an operating phase of the engine where the aircraft is beingpropelled along the ground by the thrust produced by the engine. Duringtaxi, the engine may be producing between 5% and 15% of availablethrust. In further examples, the engine may be producing between 6% and12% of available thrust. In yet further examples, the engine may beproducing between 7% and 10% of available thrust. Take-off may refer toan operating phase of the engine where the aircraft is being propelledby the thrust produced by the engine. At an initial stage within thetake-off phase, the aircraft may be propelled whilst the aircraft is incontact with the ground. At a later stage within the take-off phase, theaircraft may be propelled whilst the aircraft is not in contact with theground. During take-off, the engine may be producing between 90% and100% of available thrust. In further examples, the engine may beproducing between 95% and 100% of available thrust. In yet furtherexamples, the engine may be producing 100% of available thrust.

Climb may refer to an operating phase of the engine where the aircraftis being propelled by the thrust produced by the engine. During climb,the engine may be producing between 75% and 100% of available thrust. Infurther examples, the engine may be producing between 80% and 95% ofavailable thrust. In yet further examples, the engine may be producingbetween 85% and 90% of available thrust. In this regard, climb may referto an operating phase within an aircraft flight cycle between take-offand the arrival at cruise conditions. Additionally or alternatively,climb may refer to a nominal point in an aircraft flight cycle betweentake-off and landing, where a relative increase in altitude is required,which may require an additional thrust demand of the engine.

As used herein, cruise conditions have the conventional meaning andwould be readily understood by the skilled person. Thus, for a given gasturbine engine for an aircraft, the skilled person would immediatelyrecognise cruise conditions to mean the operating point of the engine atmid-cruise of a given mission (which may be referred to in the industryas the “economic mission”) of an aircraft to which the gas turbineengine is designed to be attached. In this regard, mid-cruise is thepoint in an aircraft flight cycle at which 50% of the total fuel that isburned between top of climb and start of descent has been burned (whichmay be approximated by the midpoint—in terms of time and/ordistance—between top of climb and start of descent. Cruise conditionsthus define an operating point of, the gas turbine engine that providesa thrust that would ensure steady state operation (i.e. maintaining aconstant altitude and constant Mach Number) at mid-cruise of an aircraftto which it is designed to be attached, taking into account the numberof engines provided to that aircraft. For example where an engine isdesigned to be attached to an aircraft that has two engines of the sametype, at cruise conditions the engine provides half of the total thrustthat would be required for steady state operation of that aircraft atmid-cruise.

In other words, for a given gas turbine engine for an aircraft, cruiseconditions are defined as the operating point of the engine thatprovides a specified thrust (required to provide—in combination with anyother engines on the aircraft—steady state operation of the aircraft towhich it is designed to be attached at a given mid-cruise Mach Number)at the mid-cruise atmospheric conditions (defined by the InternationalStandard Atmosphere according to ISO 2533 at the mid-cruise altitude).For any given gas turbine engine for an aircraft, the mid-cruise thrust,atmospheric conditions and Mach Number are known, and thus the operatingpoint of the engine at cruise conditions is clearly defined.

Purely by way of example, the forward speed at the cruise condition maybe any point in the range of from Mach 0.7 to 0.9, for example 0.75 to0.85, for example 0.78 to 0.84, for example 0.77 to 0.83, for example0.78 to 0.82, for example 0.79 to 0.81, for example on the order of Mach0.8, on the order of Mach 0.85 or in the range of from 0.8 to 0.85. Anysingle speed within these ranges may be part of the cruise condition.For some aircraft, the cruise conditions may be outside these ranges,for example below Mach 0.7 or above Mach 0.9.

Purely by way of example, the cruise conditions may correspond tostandard atmospheric conditions (according to the International StandardAtmosphere, ISA) at an altitude that is in the range of from 10000 m to15000 m, for example in the range of from 10000 m to 12000 m, forexample in the range of from 10400 m to 11600 m (around 38000 ft), forexample in the range of from 10500 m to 11500 m, for example in therange of from 10600 m to 11400 m, for example in the range of from 10700m (around 35000 ft) to 11300 m, for example in the range of from 10800 mto 11200 m, for example in the range of from 10900 m to 11100 m, forexample on the order of 11000 m. The cruise conditions may correspond tostandard atmospheric conditions at any given altitude in these ranges.

Purely by way of example, the cruise conditions may correspond to anoperating point of the engine that provides a known required thrustlevel (for example a value in the range of from 30 kN to 35 kN) at aforward Mach number of 0.8 and standard atmospheric conditions(according to the International Standard Atmosphere) at an altitude of38000 ft (11582 m). Purely by way of further example, the cruiseconditions may correspond to an operating point of the engine thatprovides a known required thrust level (for example a value in the rangeof from 50 kN to 65 kN) at a forward Mach number of 0.85 and standardatmospheric conditions (according to the International StandardAtmosphere) at an altitude of 35000 ft (10668 m).

In use, a gas turbine engine described and/or claimed herein may operateat the cruise conditions defined elsewhere herein. Such cruiseconditions may be determined by the cruise conditions (for example themid-cruise conditions) of an aircraft to which at least one (for example2 or 4) gas turbine engine may be mounted in order to provide propulsivethrust.

Furthermore, the skilled person would immediately recognise either orboth of descent and approach to refer to an operating phase within anaircraft flight cycle between cruise and landing of the aircraft.

During either or both of descent and approach, the engine may beproducing between 20% and 50% of available thrust. In further examples,the engine may be producing between 25% and 40% of available thrust. Inyet further examples, the engine may be producing between 30% and 35% ofavailable thrust. Additionally or alternatively, descent may refer to anominal point in an aircraft flight cycle between take-off and landing,where a relative decrease in altitude is required, and which may requirea reduced thrust demand of the engine.

According to an aspect, there is provided an aircraft comprising a gasturbine engine as described and/or claimed herein. The aircraftaccording to this aspect is the aircraft for which the gas turbineengine has been designed to be attached. Accordingly, the cruiseconditions according to this aspect correspond to the mid-cruise of theaircraft, as defined elsewhere herein.

According to an aspect, there is provided a method of operating a gasturbine engine as described and/or claimed herein. The operation may beat the cruise conditions as defined elsewhere herein (for example interms of the thrust, atmospheric conditions and Mach Number).

According to an aspect, there is provided a method of operating anaircraft comprising a gas turbine engine as described and/or claimedherein. The operation according to this aspect may include (or may be)operation at the mid-cruise of the aircraft, as defined elsewhereherein.

The skilled person will appreciate that except where mutually exclusive,a feature or parameter described in relation to any one of the aboveaspects may be applied to any other aspect. Furthermore, except wheremutually exclusive, any feature or parameter described herein may beapplied to any aspect and/or combined with any other feature orparameter described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described by way of example only, with referenceto the Figures, in which:

FIG. 1 is a sectional side view of a gas turbine engine;

FIG. 2 is a close up sectional side view of an upstream portion of a gasturbine engine;

FIG. 3 is a partially cut-away view of a gearbox for a gas turbineengine;

FIG. 4 is a schematic view of an aircraft including a fuel supplysystem;

FIG. 5 is a schematic view of an aircraft including a fuelcharacteristic determination system;

FIG. 6 is a close-up schematic view of the fuel characteristicdetermination system of FIG. 5 ;

FIG. 7 is schematic view of a fuel characteristic determination systemprovided within a fuel system of a gas turbine engine;

FIG. 8 is a schematic representation of a method of determining fuelcharacteristics of an aviation fuel;

FIG. 9 is a schematic view of an aircraft including another example of afuel characteristic determination system;

FIG. 10 is a close-up schematic view of the fuel characteristicdetermination system of FIG. 9 ;

FIG. 11 is schematic view of a fuel characteristic determination systemprovided within a fuel system of a gas turbine engine;

FIG. 12 is a schematic representation of another example of a method ofdetermining fuel characteristics of an aviation fuel;

FIG. 13 is a schematic view of a fuel characteristic determinationsystem having a sensor component made from the same material as one ormore seals provided in a fuel system of an aircraft;

FIG. 14 is a schematic representation of another example of a method ofdetermining fuel characteristics of an aviation fuel:

FIG. 15 is a schematic view of an aircraft including another example ofa fuel characteristic determination system;

FIG. 16 is a close-up schematic view of the fuel characteristicdetermination system of FIG. 15 ;

FIG. 17 is schematic view of another example of a fuel characteristicdetermination system provided within a fuel system of a gas turbineengine;

FIG. 18 is a schematic representation of another example of a method ofdetermining fuel characteristics of an aviation fuel:

FIG. 19 is a schematic view of an aircraft including another example ofa fuel characteristic determination system;

FIG. 20 is a close-up schematic view of the fuel characteristicdetermination system of FIG. 19 :

FIG. 21 is schematic view of another example of a fuel characteristicdetermination system provided within a fuel system of a gas turbineengine;

FIG. 22 is a schematic representation of another example of a method ofdetermining fuel characteristics of an aviation fuel;

FIG. 23 is a schematic view of an aircraft including another example ofa fuel characteristic determination system;

FIG. 24 is a schematic representation of another example of a method ofdetermining fuel characteristics of an aviation fuel;

FIG. 25 is a schematic view of an aircraft including another example ofa fuel characteristic determination system;

FIG. 26 is a schematic representation of another example of a method ofdetermining fuel characteristics of an aviation fuel;

FIG. 27 is a schematic view of an aircraft including another example ofa fuel characteristic determination system;

FIG. 28 is a schematic representation of another example of a method ofdetermining fuel characteristics of an aviation fuel;

FIG. 29 is a schematic representation of a method of operating anaircraft:

FIG. 30 is a schematic representation of another method of operating anaircraft;

FIG. 31 is a schematic representation of yet another method of operatingan aircraft;

FIG. 32 is a schematic representation of a method of generating amaintenance schedule for an aircraft;

FIG. 33 is a schematic representation of a method of maintaining anaircraft; and

FIG. 34 is a schematic view on an aircraft having a maintenance schedulegeneration system.

DETAILED DESCRIPTION OF THE DISCLOSURE

FIG. 1 illustrates a gas turbine engine 10 having a principal rotationalaxis 9. The engine 10 comprises an air intake 12 and a propulsive fan 23that generates two airflows: a core airflow A and a bypass airflow B.The gas turbine engine 10 comprises a core 11 that receives the coreairflow A. The engine core 11 comprises, in axial flow series, a lowpressure compressor 14, a high-pressure compressor 15, combustionequipment 16, a high-pressure turbine 17, a low pressure turbine 19 anda core exhaust nozzle 20. A nacelle 21 surrounds the gas turbine engine10 and defines a bypass duct 22 and a bypass exhaust nozzle 18. Thebypass airflow B flows through the bypass duct 22.

The fan 23 is attached to and driven by the low pressure turbine 19 viaa shaft 26 and an epicyclic gearbox 30.

In use, the core airflow A is accelerated and compressed by the lowpressure compressor 14 and directed into the high pressure compressor 15where further compression takes place. The compressed air exhausted fromthe high pressure compressor 15 is directed into the combustionequipment 16 where it is mixed with fuel F and the mixture is combusted.The resultant hot combustion products then expand through, and therebydrive, the high pressure and low pressure turbines 17, 19 before beingexhausted through the nozzle 20 to provide some propulsive thrust. Thehigh pressure turbine 17 drives the high pressure compressor 15 by asuitable interconnecting shaft 27. The fan 23 generally provides themajority of the propulsive thrust. The epicyclic gearbox 30 is areduction gearbox.

An exemplary arrangement for a geared fan gas turbine engine 10 is shownin FIG. 2 . The low pressure turbine 19 (see FIG. 1 ) drives the shaft26, which is coupled to a sun wheel, or sun gear, 28 of the epicyclicgear arrangement 30. Radially outwardly of the sun gear 28 andintermeshing therewith is a plurality of planet gears 32 that arecoupled together by a planet carrier 34. The planet carrier 34constrains the planet gears 32 to precess around the sun gear 28 insynchronicity whilst enabling each planet gear 32 to rotate about itsown axis. The planet carrier 34 is coupled via linkages 36 to the fan 23in order to drive its rotation about the engine axis 9. Radiallyoutwardly of the planet gears 32 and intermeshing therewith is anannulus or ring gear 38 that is coupled, via linkages 40, to astationary supporting structure 24.

Note that the terms “low pressure turbine” and “low pressure compressor”as used herein may be taken to mean the lowest pressure turbine stagesand lowest pressure compressor stages (i.e. not including the fan 23)respectively and/or the turbine and compressor stages that are connectedtogether by the interconnecting shaft 26 with the lowest rotationalspeed in the engine (i.e. not including the gearbox output shaft thatdrives the fan 23). In some literature, the “low pressure turbine” and“low pressure compressor” referred to herein may alternatively be knownas the “intermediate pressure turbine” and “intermediate pressurecompressor”. Where such alternative nomenclature is used, the fan 23 maybe referred to as a first, or lowest pressure, compression stage.

The epicyclic gearbox 30 is shown by way of example in greater detail inFIG. 3 . Each of the sun gear 28, planet gears 32 and ring gear 38comprise teeth about their periphery to intermesh with the other gears.However, for clarity only exemplary portions of the teeth areillustrated in FIG. 3 . There are four planet gears 32 illustrated,although it will be apparent to the skilled reader that more or fewerplanet gears 32 may be provided within the scope of the claimedinvention. Practical applications of a planetary epicyclic gearbox 30generally comprise at least three planet gears 32.

The epicyclic gearbox 30 illustrated by way of example in FIGS. 2 and 3is of the planetary type, in that the planet carrier 34 is coupled to anoutput shaft via linkages 36, with the ring gear 38 fixed. However, anyother suitable type of epicyclic gearbox 30 may be used. By way offurther example, the epicyclic gearbox 30 may be a star arrangement, inwhich the planet carrier 34 is held fixed, with the ring (or annulus)gear 38 allowed to rotate. In such an arrangement the fan 23 is drivenby the ring gear 38. By way of further alternative example, the gearbox30 may be a differential gearbox in which the ring gear 38 and theplanet carrier 34 are both allowed to rotate.

It will be appreciated that the arrangement shown in FIGS. 2 and 3 is byway of example only, and various alternatives are within the scope ofthe present disclosure. Purely by way of example, any suitablearrangement may be used for locating the gearbox 30 in the engine 10and/or for connecting the gearbox 30 to the engine 10. By way of furtherexample, the connections (such as the linkages 36, 40 in the FIG. 2example) between the gearbox 30 and other parts of the engine 10 (suchas the input shaft 26, the output shaft and the fixed structure 24) mayhave any desired degree of stiffness or flexibility. By way of furtherexample, any suitable arrangement of the bearings between rotating andstationary parts of the engine (for example between the input and outputshafts from the gearbox and the fixed structures, such as the gearboxcasing) may be used, and the disclosure is not limited to the exemplaryarrangement of FIG. 2 . For example, where the gearbox 30 has a stararrangement (described above), the skilled person would readilyunderstand that the arrangement of output and support linkages andbearing locations would typically be different to that shown by way ofexample in FIG. 2 .

Accordingly, the present disclosure extends to a gas turbine enginehaving any arrangement of gearbox styles (for example star orplanetary), support structures, input and output shaft arrangement, andbearing locations.

Optionally, the gearbox may drive additional and/or alternativecomponents (e.g. the intermediate pressure compressor and/or a boostercompressor).

Other gas turbine engines to which the present disclosure may be appliedmay have alternative configurations. For example, such engines may havean alternative number of compressors and/or turbines and/or analternative number of interconnecting shafts. By way of further example,the gas turbine engine shown in FIG. 1 has a split flow nozzle 18, 20meaning that the flow through the bypass duct 22 has its own nozzle 18that is separate to and radially outside the core engine nozzle 20.However, this is not limiting, and any aspect of the present disclosuremay also apply to engines in which the flow through the bypass duct 22and the flow through the core 11 are mixed, or combined, before (orupstream of) a single nozzle, which may be referred to as a mixed flownozzle. One or both nozzles (whether mixed or split flow) may have afixed or variable area.

Whilst the described example relates to a turbofan engine, thedisclosure may apply, for example, to any type of gas turbine engine,such as an open rotor (in which the fan stage is not surrounded by anacelle) or turboprop engine, for example. In some arrangements, the gasturbine engine 10 may not comprise a gearbox 30.

The geometry of the gas turbine engine 10, and components thereof, isdefined by a conventional axis system, comprising an axial direction(which is aligned with the rotational axis 9), a radial direction (inthe bottom-to-top direction in FIG. 1 ), and a circumferential direction(perpendicular to the page in the FIG. 1 view). The axial, radial andcircumferential directions are mutually perpendicular.

The fuel F provided to the combustion equipment 16 may comprise afossil-based hydrocarbon fuel, such as Kerosene. Thus, the fuel F maycomprise molecules from one or more of the chemical families ofn-alkanes, iso-alkanes, cycloalkanes, and aromatics. Additionally oralternatively, the fuel F may comprise renewable hydrocarbons producedfrom biological or non-biological resources, otherwise known assustainable aviation fuel (SAF). In each of the provided examples, thefuel F may comprise one or more trace elements including, for example,sulphur, nitrogen, oxygen, inorganics, and metals.

Functional performance of a given composition, or blend of fuel for usein a given mission, may be defined, at least in part, by the ability ofthe fuel to service the Brayton cycle of the gas turbine engine 10.Parameters defining functional performance may include, for example,specific energy; energy density; thermal stability; and, emissionsincluding particulate matter. A relatively higher specific energy (i.e.energy per unit mass), expressed as MJ/kg, may at least partially reducetake-off weight, thus potentially providing a relative improvement infuel efficiency. A relatively higher energy density (i.e. energy perunit volume), expressed as MJ/L, may at least partially reduce take-offfuel volume, which may be particularly important for volume-limitedmissions or military operations involving refuelling. A relativelyhigher thermal stability (i.e. inhibition of fuel to degrade or cokeunder thermal stress) may permit the fuel to sustain elevatedtemperatures in the engine and fuel injectors, thus potentiallyproviding relative improvements in combustion efficiency. Reducedemissions, including particulate matter, may permit reduced contrailformation, whilst reducing the environmental impact of a given mission.Other properties of the fuel may also be key to functional performance.For example, a relatively lower freeze point (° C.) may allow long-rangemissions to optimise flight profiles; minimum aromatic concentrations(%) may ensure sufficient swelling of certain materials used in theconstruction of o-rings and seals that have been previously exposed tofuels with high aromatic contents; and, a maximum surface tension (mN/m)may ensure sufficient spray break-up and atomisation of the fuel.

The ratio of the number of hydrogen atoms to the number of carbon atomsin a molecule may influence the specific energy of a given composition,or blend of fuel. Fuels with higher ratios of hydrogen atoms to carbonatoms may have higher specific energies in the absence of bond strain.For example, fossil-based hydrocarbon fuels may comprise molecules withapproximately 7 to 18 carbons, with a significant portion of a givencomposition stemming from molecules with 9 to 15 carbons, with anaverage of 12 carbons.

ASTM International (ASTM) D7566, Standard Specification for AviationTurbine Fuels Containing Synthesized Hydrocarbons (ASTM 2019c) approvesa number of sustainable aviation fuel blends comprising between 10% and50% sustainable aviation fuel (the remainder comprising one or morefossil-based hydrocarbon fuels, such as Kerosene), with furthercompositions awaiting approval. However, there is an anticipation in theaviation industry that sustainable aviation fuel blends comprising up to(and including) 100% sustainable aviation fuel (SAF) will be eventuallyapproved for use.

Sustainable aviation fuels may comprise one or more of n-alkanes,iso-alkanes, cyclo-alkanes, and aromatics, and may be produced, forexample, from one or more of synthesis gas (syngas); lipids (e.g. fats,oils, and greases); sugars; and alcohols. Thus, sustainable aviationfuels may comprise either or both of a lower aromatic and sulphurcontent, relative to fossil-based hydrocarbon fuels. Additionally oralternatively, sustainable aviation fuels may comprise either or both ofa higher iso-alkane and cyclo-alkane content, relative to fossil-basedhydrocarbon fuels. Thus, in some examples, sustainable aviation fuelsmay comprise either or both of a density of between 90% and 98% that ofkerosene and a calorific value of between 101% and 105% that ofkerosene.

Owing at least in part to the molecular structure of sustainableaviation fuels, sustainable aviation fuels may provide benefitsincluding, for example, one or more of a higher energy density; higherspecific energy; higher specific heat capacity; higher thermalstability; higher lubricity; lower viscosity; lower surface tension;lower freeze point; lower soot emissions; and, lower CO₂ emissions,relative to fossil-based hydrocarbon fuels (e.g. when combusted in thecombustion equipment 16). Accordingly, relative to fossil-basedhydrocarbon fuels, such as Kerosene, sustainable aviation fuels may leadto either or both of a relative decrease in specific fuel consumption,and a relative decrease in maintenance costs.

An aircraft 1 comprising two gas turbine engines 10 according to any ofthe examples described herein is illustrated in FIG. 4 . In thisexample, the aircraft 1 comprises two gas turbine engines 10, but inother examples it may comprise one or more gas turbine engines. Theaircraft 1 further comprises an aircraft fuel supply system located onboard the aircraft which is suitable for suppling fuel F to each of thegas turbine engines 10 to be burnt in the engine combustion equipment 16as described above. The aircraft fuel supply system is arranged toprovide fuel to an engine fuel system provided on each of the gasturbine engines 10. The engine fuel system and aircraft fuel supplysystem together form the (overall) fuel system of the aircraft 1 inwhich fuel is stored, delivered to the engine, and combusted. The fuelsystem of the aircraft includes any component which may store fuel, orthrough which fuel flows during use or during refuelling.

The aircraft fuel supply system comprises an aircraft fuel sourcearranged to contain a fuel F to be supplied to the gas turbine engines.For the purposes of the present application the term “fuel source” isunderstood to mean either 1) a single fuel tank or 2) a plurality offuel tanks which may or may not be fluidly interconnected. In thepresent example, the fuel source comprises a plurality of wing fueltanks 53, where at least one wing fuel tank is located in the port wingand at least one wing fuel tank is located in the starboard wing, and acentre fuel tank 55 located primarily in the fuselage of the aircraft 1.Each of the centre fuel tank 55 and the wing fuel tanks 53 may comprisea plurality of fluidly interconnected fuel tanks not shown in theFigures.

For balancing purposes, one or more fuel tanks 53 in the port wing maybe fluidly connected to one or more fuel tanks 53 in the starboard wingas shown by the dotted lines in FIG. 4 . This may be done either via thecentre fuel tank 55, or bypassing the centre fuel tank(s), or both (formaximum flexibility and safety). In yet other examples, the fuel sourcemay comprise a separate trim fuel tank in order to balance the aircraftduring flight (not shown in the figures).

FIG. 4 illustrates fuel being loaded onto the aircraft 1 from a fuelstorage vessel 60. The fuel storage vessel 60 may be carried by a fuelsupply vehicle (e.g. fuel tanker) or may be a fixed storage vessel fromwhich the aircraft 1 can be refuelled. The aircraft fuel systemcomprises a fuel line connection port 62 which is coupled to a fuelloading line 61 during refuelling. The fuel loading line 61 may comprisea fuel pipe of known design. The fuel line connection port 62 is fluidlycoupled with the fuel tanks 53, 55 of the aircraft 1 by a fueltransmission line or lines 63 on board the aircraft so that fuelreceived via the fuel loading line 61 is transferred and stored withinthe fuel tanks 53, 55. The fuel loading line 61 and fuel transmissionline 63 may together form a fuel supply line used to supply fuel to thefuel tanks 53, 55 on board the aircraft 1 from the fuel storage vessel60. In some examples, the fuel transmission line(s) 63 may not bepresent, with the fuel instead delivered directly from a fuel lineconnection port for each fuel tank (or set of interconnected fueltanks). The fuel loading line 61 and the storage vessel (and anyassociated control or pump components) may form a refuelling system forthe aircraft.

Fuel Characteristics

As used herein, the term “fuel characteristics” refers to inherent fuelproperties such as fuel composition, not variable properties such asvolume or temperature. Examples of fuel characteristics of a fuelinclude:

-   -   (i) the percentage of sustainable aviation fuel in the fuel;    -   (ii) the aromatic hydrocarbon content of the fuel;    -   (iii) the multi-aromatic hydrocarbon content of the fuel;    -   (iv) the percentage of nitrogen-containing species in the fuel;    -   (v) the presence or percentage of a trace species or trace        element in the fuel (e.g. a trace substance inherently present        in the fuel, or one added deliberately to act as a tracer);    -   (vi) the hydrogen to carbon ratio of the fuel;    -   (vii) the hydrocarbon distribution of the fuel;    -   (viii) the level of non-volatile particulate matter (nvPM)        emissions on combustion (e.g. on combustion for a given        combustor design, at a given operating condition (FAR, T30,        combustor mode etc));    -   (ix) the naphthalene content of the fuel;    -   (x) the sulphur content of the fuel;    -   (xi) the cycloparaffin content of the fuel;    -   (xii) the oxygen content of the fuel;    -   (xiii) the thermal stability of the fuel (e.g. thermal breakdown        temperature);    -   (xiv) the level of coking of the fuel;    -   (xv) an indication that the fuel is a fossil fuel, for example        fossil kerosene; and    -   (xvi) one or more properties such as density, viscosity,        calorific value, and/or heat capacity.

As used herein, T30, T40 and T41, and any other numbered pressures andtemperatures, are defined using the station numbering listed in standardSAE AS755, in particular:

-   -   T30=High Pressure Compressor (HPC) Outlet Total Temperature;    -   T40=Combustion Exit Total Temperature;    -   T41=High Pressure Turbine (HPT) Rotor Entry Total Temperature.

Piezoelectric Sensor

Fuel Characteristic Determination During Refuelling Using PiezoelectricSensor:

Referring to FIG. 5 , an example of a fuel characteristic determinationsystem 114 is illustrated, located on board the aircraft 1. The fuelcharacteristic determination system 114 in this example is arranged todetermine one or more fuel characteristics of fuel F being loaded ontothe aircraft 1, those characteristics being any of those described orclaimed herein.

Further details of the fuel characteristic determination system 114 areillustrated in FIG. 6 . The fuel characteristic determination system 114generally comprises a sensor 115 which comprises a piezoelectric crystal116. The piezoelectric crystal 116 is arranged so that it is exposed tofuel F which is flowing through the sensor 115. In the present example,fuel being loaded onto the aircraft 1 is passed through the sensor 115so that the piezoelectric crystal 116 is exposed to the fuel. The sensor115 comprises a fuel inlet 115 a and a fuel outlet 115 b, and a fuelconduit 115 c arranged in fluid communication between them. In thepresent example, the sensor 115 is located on board the aircraft 1 andis arranged to receive fuel F flowing through the fuel transmission line63 i.e. fuel flowing from the fuel line connection port 62 to the fueltanks 53, 55. In the present example, the sensor 115 is arranged suchthat all of the fuel flowing through the transmission line 63 is inputto the sensor inlet 115 a, flows through the fuel conduit 115 c, and isoutput via the fuel outlet 115 b to continue to the fuel tanks (i.e. thesensor is connected in series between the fuel line connection port 62and the fuel tanks). In other examples, the sensor 115 may be connectedin parallel such that only some of the fuel is redirected from thetransmission line 63, flows through the sensor, and then is returned tothe fuel transmission line 63 or to the fuel tanks.

The sensor 115 is arranged to measure a vibration parameter of thepiezoelectric crystal 116. The vibration parameter may be a vibrationalmode of the piezoelectric crystal such as a resonant frequency at whichthe crystal oscillates. In the present example, the sensor comprises aresonant frequency measurement module 116 a arranged to measure aresonant frequency of the crystal by applying voltage to an electrodenear or on the crystal 116. This causes the crystal 116 to distort in anelectric field created by the voltage. Once the field is removed, thepiezoelectric crystal 116 generates an electric field as it returns toits previous shape, generating a voltage. This results in the crystalbehaving like an RLC circuit, composed of an inductor, capacitor andresistor with a corresponding resonant frequency as would be known inthe art.

The inventors have determined that the resonant frequency of the crystal116 varies according to deposits formed on its surface by the fuel towhich it is exposed. By measuring the resonant frequency or othervibration parameter (or change thereof overtime) the amount of surfacedeposition on the crystal surface can be determined. In the presentexample, the fuel characteristic determination system 114 furthercomprises a fuel characteristic determination module 117 arranged todetermine one or more fuel characteristics of the fuel based on thevibration parameter. The fuel characteristic determination module 117receives the vibration parameter from the sensor 115 such that the oneor more fuel characteristics can be calculated.

In some examples, the surface deposits may be cleared between differentuses of the fuel characteristic determination system, e.g. by thecrystal being exposed to a different type of fuel that clears anydeposits from the surface of the crystal. In other examples, the sensormay include means for cleaning the surface of the piezoelectric crystal,or means to allow replacement of the crystal, between uses (e.g. betweenflights of the aircraft). The may allow the characteristics of the fuelfor each use of the aircraft to be determined. In yet other examples,the surface deposits may not be removed between uses, in which case thesensor 115 is arranged to determine a maximum change in the vibrationalparameter since the piezoelectric crystal was last cleaned, reset orreplaced. This may allow a build-up of surface deposits over multipleuses of the aircraft (e.g. multiple flights) to be determined.

For example, a fuel characteristic determined based on the vibrationalparameter may include an oxygen content of the fuel, a thermal stabilityof the fuel and/or a coking level of the fuel. The amount of surfacedeposit formed and detected on the piezoelectric crystal will have adependence on these characteristics/properties of the fuel allowing themto be determined based on measurement of the vibrational parameter. Inother examples, other fuel characteristics may be determined or inferredbased on the measured vibrational parameters or other fuelcharacteristics determined from the vibrational parameters. For example,the hydrocarbon distribution of the fuel, the percentage of SAF withinthe fuel or the aromatic hydrocarbon content in the fuel (e.g.percentage by mass or volume) may be determined.

The fuel characteristic determination module 117 is arranged to transmitthe determined one or more fuel characteristics to the electronic enginecontroller EEC 42 (or other control system of the aircraft). In someexamples, the determination module 117 may be part of the EEC 42 of anyone or more of the gas turbine engines of the aircraft. Once received atthe EEC the fuel characteristics can be used to provide information onthe fuel that is being provided from the fuel tanks to the engine suchthat operation of the gas turbine engine(s) can be adapted accordingly.In yet other examples, the determination module 117 may be part of thesensor unit 115, which is in communication with the EEC.

In order to determine the one or more fuel characteristics, thedetermination module 117 may be arranged to compare a measuredvibrational parameter to a look-up table of expected vibrationalparameter values of fuels with known fuel characteristics to determinethe corresponding characteristics of the fuel to which the crystal hasbeen exposed. This may allow a range of fuel characteristics to bedetermined based on a measurement of the surface deposit formed on thepiezoelectric crystal. In yet other examples, the determination modulemay determine a level of surface deposition formed on the crystal usingthe measured resonant frequency of the crystal, and then compare thedetermined surface deposition level to values in a look-up table. Theskilled person will understand that this is equivalent to comparing theresonant frequency.

In some examples, an absolute value of the vibration parameters may bemeasured and used by the determination module 117 to determine the oneor more fuel characteristics. In other examples, a change in thevibrational parameter may be used. For example, a deviation from anexpected value if no surface deposit is present may be measured, or achange of the vibration parameter over time measured. This may allow aslow build-up of surface deposits on the crystal to be measured and usedto infer the one or more fuel characteristics. Using this method thecharacteristics of the fuel being used by the aircraft over an extendedperiod of time may be determined.

The determination module 117 may further base the fuel characteristicdetermination on one or more performance parameters of the engine or theaircraft. This may include a temperature of the fuel to which thesurface of the piezoelectric crystal has been exposed. This may allowthe expected vibrational parameters formed by fuels of knowncharacteristics at known temperatures to be compared to the measureddata in order to determine the fuel characteristics. In other examples,other operating parameters may be used by the determination module 117so that a relevant fuel characteristics can be reliably determined.

In the examples described above the fuel characteristic determinationsystem 114 is located on board the aircraft 1 so that fuel within thefuel transmission line 63 flows through it before reaching the fueltanks 53, 55. In other examples, the fuel characteristic determinationsystem 114 may be arranged to use fuel within, or sampled from, one ofthe fuel tanks 53, 55. In such an example, the sensor 115 may locatedwithin the fuel tank so that fuel may be exposed to piezoelectriccrystal 116. The sensor may therefore comprise a piezoelectric crystalmounted to an interior wall of the fuel tank such that it is exposed tofuel within the tank. In other examples, fuel may be sampled from a fueltank aboard the aircraft and passed through the sensor 115.

In other examples, the fuel characteristic determination system 114, orat least part of it, may be located separately from the aircraft 1. Forexample, it may be included in the fuel loading line 61 so that it maydetect properties of the fuel before it reaches the aircraft. In someexamples, the sensor 115 may be located within the fuel loading line, orelsewhere separately from the aircraft 1, and arranged to transmit avibrational parameter to a determination module 117 located on board theaircraft 1, where it can be communicated to a control module of theengine (e.g. the EEC) or aircraft. In yet other examples, the fuelcharacteristic determination system 114 may be located entirely outsideof the aircraft. In such an example, the fuel characteristicdetermination module 117 may determine a fuel characteristic which isthen communicated to the aircraft 1 (e.g. to a control module of theengines or engines 10). In this example, a data transfer link may beprovided (e.g. a wireless or wired data connection) and may be used tocommunicate the fuel characteristics to the aircraft from the fuelcharacteristic determination system 114. In some examples, the datatransfer may be done manually by a user, e.g. a technician or otheroperator of the system may obtain the fuel characteristics from thedetermination system 114 and manually provide them to a control moduleon board the aircraft.

Fuel Characteristic Determination within Engine Using PiezoelectricSensor

FIG. 7 illustrates another example of the fuel characteristicdetermination system 114 described above. The fuel characteristicdetermination system of FIG. 7 similarly comprises a sensor 115 and fuelcharacteristics determination module 117 arranged to measure one or morefuel characteristics based on a vibrational parameter (e.g. resonantfrequency) of a piezoelectric crystal exposed to fuel within the sensor.In this example, the fuel characteristic determination system 114 isarranged to measure the effects of the piezoelectric crystal 116 beingexposed to fuel during operation of the gas turbine engine 10.

FIG. 7 shows a schematic view part of the fuel system of the aircraftand the combustion equipment 16 of the gas turbine engine 10. Thecombustion equipment 16 comprises a plurality of fuel nozzles (not shownin FIG. 7 ) arranged to inject fuel into a combustion can. Fuel isprovided to the combustion equipment 16 by a fuel delivery regulator 107under the control of the EEC 42. Fuel is delivered to the fuel deliveryregulator 107 by a fuel pump 108 from a fuel source 109 on board theaircraft 1 (e.g. one or more fuel tanks 53, 55 as described above). Thefuel delivery regulator 107 and combustion equipment 16 may be of knowndesign, and may be arranged for staged (lean-burn) combustion orrich-burn combustion.

In this example, a vibrational parameter is measured for fuel as it isbeing used by the engine. The sensor 115 in this example is arranged tomeasure the effect of fuel exposed to the piezoelectric crystal 116 atany point within the fuel system on board the aircraft that is upstreamof the combustion equipment 16 (e.g. upstream of the fuel nozzles of thecombustion equipment 16) and downstream of the fuel source 109 fromwhich the fuel is supplied (e.g. downstream of the one or more fueltanks 53, 55 forming the fuel source). In some examples, the sensor 115is located at a point within the engine fuel system such as in a fuelconduit within or forming part of the gas turbine engine 10 (rather thanbeing on the aircraft 1 to which the gas turbine engine 10 is mounted).In some examples, it is located at a point immediately before the fuelenters the combustor 16. In yet other examples, the sensor 115 islocated at a point within the aircraft fuel supply system e.g. before itenters part of the gas turbine engine 10.

In the presently described example, the engine fuel system furthercomprises a heat management system having a heat exchanger 118. The heatexchanger is arranged to transfer heat between the fuel and an oilsystem of the engine, e.g. transfer heat from the oil-system into thefuel so as to cool the oil and warm the fuel. As illustrated in FIG. 7 ,the sensor 115 is located within the engine fuel system at a point atwhich fuel has been heated by the heat exchanger 118 during operation ofthe gas turbine engine 10. The inventors have observed that duringoperation of the engine 10 heating of the fuel can result in thermalbreakdown of the fuel that causes deposits to be formed on the surfaceof the piezoelectric crystal 116. For example, coking of the fuel canoccur when the fuel is heated by the heat exchanger of the engine toprovide cooling. The inventors have determined that the amount ofdeposit formed, and hence the vibrational parameters of the crystal, aredependent on the susceptibility of the fuel to thermal breakdown. Assusceptibility of the fuel to thermal breakdown varies between differentfuels, a measurement sensitive to the thermal breakdown can be used todetermine the characteristics of the fuel being used by the engine. Forexample, the SAF content of the fuel may be determined, as a SAF richfuel may be associated with a lower degree of thermal breakdown. In thepresent example, the fuel characteristic determination system istherefore arranged to determine characteristics of the fuel based on themeasured level of thermal breakdown of the fuel.

In the example of FIG. 7 the fuel characteristic determination module117 is arranged to compare the measured vibrational parameter (e.g.resonant frequency) or level of surface deposition to a look-up table ofvalues corresponding to those expected for fuels of knowncharacteristics. The fuel characteristic determination module 117 mayfurther base the comparison with the look-up table on a temperature ofthe fuel to which the piezoelectric crystal is exposed with temperaturevalues in the look-up table at which the changes in vibration parametersare expected for known fuel characteristics. Thus the measured surfacedeposit occurring at a certain temperature can be referenced in thelook-up table to that of a known fuel at the same or similartemperature.

In the presently described examples, the fuel characteristicdetermination module 117 is arranged to determine the one or more fuelcharacteristics based only on the determined vibration parameter of thepiezoelectric crystal 116. In other examples, the determination module117 may be arranged to combine the vibration parameter information withinputs from other sensors or other methods of determining fuelcharacteristics disclosed herein. This may allow a greater range oftypes of fuel characteristic to be inferred, or improve the accuracy ofthe fuel characteristic determination.

FIG. 8 illustrates a method 1014 of determining one or more fuelcharacteristic of an aviation fuel suitable for powering a gas turbineengine of an aircraft that can be performed by the fuel characteristicdetermination systems 114 shown in FIGS. 5, 6 and 7 . The method 1014comprises exposing 1015 the surface of a piezoelectric crystal to thefuel; measuring 1016 a vibration parameter of the piezoelectric crystal;and determining 1017 one or more fuel characteristics of the fuel basedon the vibration parameter. Any of the features described above inconnection with FIGS. 5, 6 and 7 can be incorporated in the method ofFIG. 8 , and so will not be repeated here.

Seal Swell Sensor

Fuel Characteristic Determination During Refuelling Using Swell Sensor

Referring to FIG. 9 , another example of a fuel characteristicdetermination system 119 is illustrated, located on board the aircraft1. The fuel characteristic determination system 119 in this example isarranged to determine one or more fuel characteristics of fuel F beingloaded onto the aircraft 1, those characteristics being any of thosedescribed or claimed herein.

Further details of the fuel characteristic determination system 119 areillustrated in FIG. 10 . The fuel characteristic determination system119 generally comprises a detection device 120 that is located withinthe fuel system of the aircraft. The detection device 120 comprises asensor component 121 formed from a seal material. In some examples asdescribed later, the seal material is formed from the same material asone or more seals provided in the fuel supply system. The one or moreseals are exposed to the fuel during use of the fuel system of theaircraft, and are configured to swell upon contact with the fuel. Inother examples, the seal material may be different from those usedelsewhere in the fuel system of the aircraft or engine. The sealmaterial may be nitrile seal material (e.g. nitrile rubber or Buna-N).The seals may be any seals that are exposed to fuel within the fuelsystem of the aircraft.

In the present example, fuel being loaded onto the aircraft is passedthrough the detection device 120 and is exposed to the sensor component121. The detection device 120 comprises a fuel inlet 120 a and a fueloutlet 120 b, and a fuel conduit 120 c arranged in fluid communicationbetween them. In the present example, detection device 120 is located onboard the aircraft and is arranged to receive fuel flowing through thefuel transmission line 63 i.e. fuel flowing from the fuel lineconnection port 62 to the fuel tanks 53, 55. The sensor is arranged suchthat all of the fuel flowing through the transmission line 63 is inputto the inlet 120 a, flows through the fuel conduit 120 c, and is outputvia the fuel outlet 120 b to continue to the fuel tanks (i.e. thedetection device is connected in series between the fuel line connectionport 62 and the fuel tanks). In other examples, the detection device 120may be connected in parallel such that only some of the fuel isredirected from the transmission line 63, flows through the detectiondevice 120, and then is returned to the fuel transmission line 63 or tothe fuel tanks.

The fuel characteristic determination system further comprises a sensor122 arranged to measure a swell parameter of the seal material fromwhich the sensor component 121 is made. The swell parameter indicatesthe degree to which the seal material has changed in shape and/or sizein response to the exposure of fuel F to its surface. The inventors havedetermined that the degree of swell of the sensor component 121 variesaccording the properties of the fuel F to which it is exposed. Forexample, the seal material from which the sensor component 121 is mademay expand or contract according to the characteristics of the fuel. Bymeasuring expansion or contraction of the seal material (or changethereof over time) various properties of the fuel F may be determinedbased on the corresponding swell parameter.

Referring again to FIG. 10 , the sensor component 121 of the presentexample is fixedly mounted within the detection device 120 via a fixingstructure 123 a. As can be seen schematically in FIG. 10 , part of thesensor component 121 is fixedly coupled to the detection device 120 sothat it is free to expand or contract in response to exposure to thefuel. The sensor 122 further comprises a gauge 123 b relative to whichthe seal material is fixedly mounted. In the present example, the gauge123 b is fixedly mounted within the detection device 120 via a gaugefixing structure 123 c. The gauge 123 b is arranged to detect movementof part of the sensor component 121 resulting from its expansion orcontraction. The gauge 123 b may be a pressure sensing device arrangedto detect a pressure exerted on it by the sensor component 121. As canbe seen in FIG. 10 the sensor component is constrained between thefixing structure 123 a and the gauge 123 b such that any change in itssize will result in a change in the pressure applied to the gauge 123 b.

In other examples, other methods of detecting the expansion orcontraction of the sensor component 121 can be used. For example, thegauge may be arranged to detect a change in physical shape of the sensorcomponent. The gauge may, for example, be arranged to detect a change inthe physical position of an unconstrained surface of the sensorcomponent to determine the level of expansion or contraction.

The fuel characteristic determination system 119 of the present examplefurther comprises a fuel characteristic determination module 124arranged to determine one or more fuel characteristics of the fuel Fbased on the swell parameter. The fuel characteristic determinationmodule 124 receives the swell parameter from the sensor 122 such thatthe one or more fuel characteristics can be calculated.

The fuel characteristic determination system 119 of the present examplemay be arranged to determine any one or more of the fuel characteristicsdefined or claimed herein based on the swell parameter. For example, thefuel characteristic determination system 119 may be arranged todetermine a hydrocarbon distribution of the fuel, for example a propertyrelated to the aromatic hydrocarbon content of the fuel. The one or morefuel characteristics determined may therefore include the percentage bymass or volume of aromatic hydrocarbon compounds in the fuel. Theinventors have determined that the aromatic content of the fuel isrelated to the swell of the seal material, with different levels ofaromatic compounds resulting in different levels of swell. Measurementof the swell parameter may therefore be used to determine the aromaticcontent of the fuel. Other fuel characteristics may however bedetermined based on the swell parameter. For example, other fuelcharacteristics such as the percentage of sustainable aviation fuelpresent within the fuel may be determined (e.g. by being inferred fromthe level of aromatic content, or because of the resulting change insize of the seal material compared to a fuel of known characteristics).In other examples, the fuel characteristic may be the cycloparaffincontent of the fuel.

The fuel characteristic determination module 124 is arranged transmitthe determined one or more fuel characteristics to the electronic enginecontroller EEC 42 (or other control module of the aircraft). In someexamples, the determination module 124 may be part of the EEC 42 of anyone or more of the gas turbine engines of the aircraft. Once received atthe EEC the fuel characteristics can be used to provide information onthe fuel that is being provided from the fuel tanks to the engine suchthat operation of the gas turbine engine(s) can be adapted accordingly.In yet other examples, the determination module 124 may be part of thedetection device 120, which is in communication with the EEC.

In order to determine the one or more fuel characteristics, thedetermination module 124 may be arranged to compare a swell parameter toa look-up table of expected swell parameter values of fuels with knownfuel characteristics to determine the corresponding characteristics ofthe fuel to which the sensor component has been exposed. This may allowa variety of fuel characteristics to be determined.

In some examples, an absolute value of the swell parameters may bemeasured and used by the determination module 124 to determine the oneor more fuel characteristics. In other examples, a change in the swellparameter may be used. For example, a deviation from an expected valuemay be measured, or a change of the swell parameter over time measured.This may allow a slow change in the expansion or contraction of the sealmaterial to be measured and used to determine the one or more fuelcharacteristics. Using this method, the characteristics of the fuelbeing used by the aircraft over an extended period of time may bedetermined.

In the examples described above the fuel characteristic determinationsystem 119 is located on board the aircraft 1 so that fuel within thefuel transmission line 63 flow through it before reaching the fuel tanks53, 55. In other examples, the fuel characteristic determination system119 may be arranged to use fuel with, or sampled from, one of the fueltanks 53, 55. In such an example, the detection device 120 may belocated within the fuel tank so that fuel may be exposed to the sensorcomponent 121. The detection device may therefore comprise the sensorcomponent mounted to an interior wall of the fuel tank such that it isexposed to fuel within the tank. In such an example, the inlet 120 a,outlet 120 b and conduit 120 c structure may not be required. The fixingstructure 123 a, gauge fixing structure 123 c and gauge 123 b howevermay still be provided. The In other examples, fuel may be sampled fromthe tank and passed through the detection device 120.

In other examples, the fuel characteristic determination system 119, orat least part of it, may be located separately from the aircraft 1. Forexample, it may be included in the fuel loading line 61 so that it maydetect properties of the fuel before it reaches the aircraft. In someexamples, the detection device 120 may be located within the fuelloading line, or elsewhere separately from the aircraft 1, and arrangedto transmit a swell parameter to a fuel characteristic determinationmodule 124 located on board the aircraft 1, where it can be communicatedto a control module of the engine (e.g. the EEC) or the aircraft. In yetother examples, the fuel characteristic determination system 119 may belocated entirely outside of the aircraft. In such an example, the fuelcharacteristic determination module 124 may determine a fuelcharacteristic which is then communicated to the aircraft 1 (e.g. to acontrol module of the engines or engines 10). In this example, a datatransfer link may be provided (e.g. a wireless or wired data connection)and may be used to communicate the fuel characteristics to the aircraftfrom the fuel characteristic determination system 119. In some examples,the data transfer may be done manually by a user, e.g. a technician orother operator of the system may obtain the fuel characteristics fromthe fuel characteristic determination system 119 and manually providethem to a control module on board the aircraft.

Fuel Characteristic Determination During Engine Operation Using SwellSensor

FIG. 11 illustrates another example of the fuel characteristicdetermination system 119 described above. The fuel characteristicdetermination system of FIG. 11 similarly comprises a detection device120 and determination module 124 arranged to measure one or more fuelcharacteristics based on a swell parameter. The swell parameter isdetermined by measuring the expansion or contraction of a sensorcomponent exposed to fuel within the detection device 120. In thisexample, the fuel characteristic determination system 119 is arranged tomeasure the effects of fuel being exposed to the sensor component 121during operation of the gas turbine engine 10.

FIG. 11 shows a schematic view of part of the fuel system of theaircraft and the combustion equipment 16 of the gas turbine engine 10.The combustion equipment 16 comprises a plurality of fuel nozzles (notshown in FIG. 11 ) arranged to inject fuel into a combustion can. Fuelis provided to the combustion equipment 16 by a fuel delivery regulator107 under the control of the EEC 42. Fuel is delivered to the fueldelivery regulator 107 by a fuel pump 108 from a fuel source 109 onboard the aircraft 1 (e.g. one or more fuel tanks as described above).The fuel delivery regulator 107 and combustion equipment 16 may be ofknown design, and may be arranged for staged (lean-burn) combustion orrich-burn combustion.

In this example, a swell parameter is measured for fuel as it is beingused by the engine. The detection device 120 in this example is arrangedto measure the effect of fuel exposed to the sensor component 121 at anypoint within the fuel system of the aircraft that is upstream of thecombustion equipment 16 (e.g. upstream of the fuel nozzles of thecombustion equipment 16) and downstream of the fuel source 109 fromwhich the fuel is supplied (e.g. downstream of the one or more fueltanks 53, 55 forming the fuel source). In some examples, the detectiondevice 120 is located at a point within the engine fuel system such asin a fuel conduit within or forming part of the gas turbine engine 10(rather than being on the aircraft 1 to which the gas turbine engine 10is mounted). In some examples, it is located at a point immediatelybefore the fuel is combusted (e.g. immediately before entering thecombustor). In yet other examples, the detector device 120 is located ata point within the aircraft fuel supply system e.g. before it enterspart of the gas turbine engine 10.

In the example shown in FIG. 11 , the detector device 120 is arrangedsuch that all of the fuel flowing to the combustor passes through it(e.g. it is arranged in series). In other examples, only some of thefuel may pass through the detector device 120. For example, the detectordevice may be located in bleed line of the fuel system at which fuel issampled from fuel supply to the combustor (e.g. before or after fuelmixing).

In the presently described examples (e.g. as shown in FIGS. 9, 10 and 11), the fuel characteristic determination module 124 is arrangeddetermine the one or more fuel characteristics based only on thedetermined swell parameter of the seal material component 121. In otherexamples, the determination module 124 may be arranged to combine theswell parameter information with inputs from other sensors or othermethods of determining fuel characteristics disclosed herein. This mayallow a greater range or types of fuel characteristic to be inferred, orimprove the accuracy of the fuel characteristic determination.

In any of the examples described above, the fuel characteristicdetermination system is arranged to generate an alert signal if theswell parameter is beyond an alert threshold or outside an acceptablerange i.e. above a first upper threshold or below a second lowerthreshold. The alert signal may be generated by the fuel characteristicdetermination module 124, or may be generated at the sensor 122. Thealert signal may be generated based on a comparison to an alertthreshold e.g. if the swell parameter exceeds or is below the alertthreshold. For example, if the swell parameter indicates that the swellof the material has fallen below a safety threshold, the alert signalmay be generated to indicate that seals made from the same or similarmaterial on board the aircraft may have contracted to a degree thatwould affect their sealing ability. This may be used to provide anindication that there is a risk of insufficient sealing by seals thatrely on swelling in size on exposure to fuel during use.

FIG. 12 illustrates a method 1021 of determining one or more fuelcharacteristics of an aviation fuel suitable for powering a gas turbineengine of an aircraft. The method 1021 can be performed by the fuelcharacteristic determination systems 119 shown in FIGS. 9, 10 and 11 .The method 1021 comprises exposing 1022 the surface of a componentformed from a nitrile seal material (e.g. sensor component 121);measuring 1023 a swell parameter of the seal material; and determining1024 one or more fuel characteristics of the fuel based on the swellparameter. The method may further comprise generating 1024 a an alertsignal if the swell parameter is outside of an alert threshold (e.g.above or below the threshold). Any of the features described above inconnection with FIGS. 9, 10 and 11 can be used in the method 1021, andso will not be repeated here.

As discussed more generally below, the method 1021 of determining one ormore fuel characteristics may be part of a method of operating anaircraft 1065. In such a method, the aircraft, or more specifically agas turbine engine mounted to the aircraft, is operated according to theone or more fuel characteristics determined using the method 1021. Insome examples, operating the aircraft according to the fuelcharacteristics may include providing a fuel having at least onedifferent characteristic to those measured to the combustor of the gasturbine engine. This may be done by refuelling using fuel havingdifferent characteristics the next time the aircraft is refuelled, ormay comprise controlling the supply of fuel from different fuel sourceson board the aircraft. For example, fuel from an alternative fuel tankholding fuel of a different characteristic may be used, or a differentblend of fuels used from two or more different sources.

The supply of a fuel having a different characteristic may comprise:providing fuel with a relatively higher aromatic content compared towhich the fuel characteristics were determined; ii) providing fuel witha lower SAF content compared to which the fuel characteristics weredetermined; and/or iii) providing fossil kerosene fuel. This may allow afuel causing a greater expansion of the seal material to be used, whichmay aid the performance of other seals provided in the fuel system ofthe aircraft by increasing their swell response to the fuel. In otherexamples, other operating parameters of the aircraft or gas turbineengine may be modified in response to the determined fuelcharacteristics.

Fuel Characteristic Determination Using a Sensor Component Made from aMaterial Matching that of Other Seals on Board the Aircraft:

In the examples described above the seal material of the sensorcomponent 121 is a nitrile seal material. The specific material chosenfor use as a sensor may be independent of any other types of sealmaterial used aboard the aircraft. The material may therefore be chosenaccording to a desired response to certain fuel characteristics to bedetermined. In other examples, however, the seal material is not limitedto being a specific type of material, but is chosen such that it is thesame as the material of one or more seals that are used on board theaircraft.

An example in which the seal material matches that of the aircraft isillustrated in FIG. 13 . Features common to FIGS. 9, 10 and 11 arelabelled accordingly. A propulsion system 10 a for an aircraft isillustrated, having gas turbine engines 10 and a fuel system 10 bcomprising fuel tanks 53, 55 arranged to contain fuel for supply to thegas turbine engine. In other examples, any number of gas turbine enginesand fuel tanks may be provided. The fuel system 10 b may correspond tothe “fuel system of the aircraft” introduced previously, including theengine fuel system and aircraft fuel supply system.

The fuel system 10 b further comprises seals 125 (only one of which islabelled). The seals in the present examples are arranged to seal fuelconduits extending between the centre (fuselage) fuel tank 55 and thewing fuel tanks 53 and the gas turbine engines 10. The seals 125 areexposed to fuel from the fuel tanks 53, 55 during use of the aircraft.In other examples, the seals may be provided for any other use withinthe fuel system of the aircraft which are exposed to fuel during use.They may, for example, be part of a fuel pump. The seals 125 are of atype arranged to swell in response to exposure to fuel during use inorder to providing sealing, and may be formed from a nitrile sealmaterial.

The propulsion system further comprises a detection device 120 locatedwithin the fuel system of the aircraft 1. The detection device mayinclude the same components as that described above in connection withFIGS. 10 and 11 , and includes a sensor component 121 which is exposedto the fuel within the fuel system. The sensor component is formed fromthe same material as the seals 125 provided in the fuel system of theaircraft. The detection device 120 is arranged to detect or determine aswell parameter of the seal material component as described above.

The inventors have determined that by detecting the swell of a sensorcomponent made from the same material as the seals otherwise provided inthe fuel system of the aircraft the degree of swell of the sealsthemselves can be determined. This allows indirect monitoring of thedegree of swell of the seals, without having to disassemble componentsfor inspection. As the seals 125 are intended to swell in order toprovide sufficient sealing, measuring of the swell of the sensorcomponent 121 can provide an indirect measurement of their performance.For example, a reduction in the swell of the sensor component 121 belowa threshold may indicate that the other seals within the fuel system ofthe aircraft would not have a sufficient degree of swell to ensuresealing. The detection device may be arranged to generate an alertsignal if the swell parameter falls below an alert threshold to indicatethat insufficient swell of the seals 125 may be occurring.

The detection device 120 may be provided such that it received fuelduring use of the gas turbine engine. It may, for example, be located asdescribed in connection with FIG. 11 .

In the present example, the propulsion system further comprises a fuelcharacteristic determination module 124 as described above to allow fuelcharacteristics to be determined based on the swell parameter. This canbe achieved in the same way as described above in connection with FIGS.9, 10 and 11 . Any feature described above in connection with thosefigures may be used in combination with the example shown in FIG. 13 .

In other examples, the fuel characteristic determination module 124 maynot be provided. In such examples, the fuel swell parameter may be usedto indicate swell of the seals 125, rather than allow fuelcharacteristics to be determined.

FIG. 14 illustrates a method 1025 that may be performed using theexample of FIG. 13 . The method 1025 comprises: exposing 1028 one ormore seals of the fuel supply system of the aircraft to fuel within thefuel supply system; exposing 1027 the sensor component 121 made from thesame material as the one or more seals to the fuel, the component beinglocated within the fuel system of the aircraft; and measuring 1028 aswell parameter of the seal material. The method 1025 may furthercomprise determining 1029 a one or more fuel characteristics of the fuelbased on the swell parameter and/or generating 1029 b an alert signal ifthe swell parameter exceeds an alert threshold. Any of the featuresdescribed above in connection with FIGS. 9 to 13 may be incorporatedinto the method of FIG. 14 .

Trace Substance Sensor

Fuel characteristic determination during refuelling using a tracesubstance sensor-Referring to FIG. 15 , another example of a fuelcharacteristic determination system 126 is illustrated, located on boardthe aircraft 1. The fuel characteristic determination system 126 in thisexample is arranged to determine one or more fuel characteristics offuel being loaded onto the aircraft 1, those characteristics being anyof those described or claimed herein. In this example, the fuelcharacteristic is determined based on the detection of trace substancesin the fuel F.

Further details of the fuel characteristic determination system 126 areillustrated in FIG. 16 . The fuel characteristic determination system126 generally comprises a trace sensor 127 which is arranged to measureone or more trace substance parameters of the fuel being supplied to theaircraft fuel tanks 53, 55.

The trace substance parameters measured by the sensor 127 are eachassociated with a respective trace substance in the fuel. By tracesubstance we mean a substance in the fuel whose concentration (or othermeasure of amount) is very low i.e. it is present in a trace amount. Atrace substance may include any type of substance present in traceamounts, for example trace chemical elements, compounds, molecules etc.The trace substance may be a substance which, based on its presence,absence, or amount in the fuel, one or more characteristics of the fuelcan be directly determined or inferred. The trace substance parameter(s)may in some examples indicate the presence of a concentration of theassociated trace substance in the fuel. They may therefore be aconcentration measured as the mass fraction of a trace substance in thefuel (e.g. a concentration in parts-per-million). In other examples, thetrace substance parameter(s) may indicate the absence or presence (e.g.within measurement limits) of an associated trace substance in the fuel.

The fuel characteristic determination system 126 further comprises adetermination module 128 arranged to determine one or more fuelcharacteristics of the fuel based on the trace substance parameters. Thedetermination module 128 is arranged to receive the trace substanceparameter from the trace sensor 127 such that the one or more fuelcharacteristics can be calculated.

The inventors have determined that by measurement of trace substancespresent or absent within a fuel the characteristics of that fuel can bedetermined either directly or indirectly.

In one example, the trace substance parameter measured by the tracesensor 127 is associated with a sulphur content of the fuel. The tracesubstance parameter may in this example be a concentration (or othermeasurement of amount) of sulphur molecules within the fuel. Theinventors have observed that for all SAF production pathways theresulting fuel is characterised by having an almost complete absence ofsulphur molecules. The inventors have determined that this can be usedas an indicator of the amount of SAF content of the fuel by measurementof the concentration of sulphur molecules by the trace sensor 127. Thefuel characteristic determination system 126 may in this example comparethe measured sulphur content of the fuel to that corresponding to fossilderived fuel. This may allow the system 126 to determine a percentage ofSAF in the fuel F being supplied to the aircraft. As a SAF containsalmost completely no sulphur molecules, the measured concentration canbe compared to a sulphur concentration of typically around 500 ppm (andup to 3000 ppm) of elemental sulphur within a fossil aircraft fuel. Bymeasuring a reduction in the sulphur content the relative amount of SAFpresent in the fuel as a percentage of the total fuel (e.g. thepercentage SAF per unit mass or volume) can therefore be derived (forexample using knowledge of the amount of sulphur (e.g. in ppm) withinthe fossil fuel to infer from a measured amount of sulphur (e.g. in ppm)within a fuel mixture what the percentage of SAF is within thatmixture). In some examples, the trace substance parameters may indicatean absence (within a measurable limit) of sulphur molecules within thefuel F, based on which the fuel characteristic determination module 128may determine that no fossil kerosene (within measurable limits) ispresent with the fuel.

In the example illustrated in FIG. 18 , the sensor 127 comprises afluorescence detection device arranged to measure the amount of sulphurmolecules within the fuel F. As can be seen in FIG. 16 , fuel beingloaded onto the aircraft is passed through the sensor 127 where a tracesubstance measurement is performed on the fuel. The sensor 127 comprisesa fuel inlet 127 a and a fuel outlet 127 b, and a fuel conduit 127 carranged in fluid communication between them. In the present example,the sensor 127 is located on board the aircraft and is arranged toreceive fuel flowing through the fuel transmission line 63 i.e. fuelflowing from the fuel line connection port 62 to the fuel tanks 53, 55.The fluorescence detection device may provide a reliable method fordetecting the concentration of sulphur molecules in the fuel. Othersensors or measurement techniques may however be used.

The fluorescence detection device comprises an excitation source 129 a,such as an LED, which is arranged to emit radiation into the fuel F. Thefluorescence detection device further comprises a detector 129 barranged to detect emitted fluorescent light. The detector 129 b may bein communication with a processor module 129 c configured to process afluorescence signal from the detector 129 c and calculate a tracesubstance parameter accordingly.

In the present example, the sensor 127 is arranged such that all of thefuel flowing through the transmission line 63 is input to the sensorinlet 127 a, flows through the fuel conduit 127 c, and is output via thefuel outlet 127 b to continues to the fuel tanks (i.e. the sensor isconnected in series between the fuel line connection port 62 and thefuel tanks). In other examples, the sensor 127 may be connected inparallel such that only some of the fuel is redirected from thetransmission line 63, flows through the sensor, and then is returned tothe fuel transmission line 63 or to the fuel tanks. In some examples,the sensor may be a microfluidic (lab-on-chip) device through which asmall sample of fuel may be passed to perform a measurement of a tracesubstance parameter.

The fuel characteristic determination module 128 is arranged transmitthe determined one or more fuel characteristics to the electronic enginecontroller EEC 42 or other control module of the aircraft. In someexamples, the fuel characteristic determination module 128 may be partof the EEC 42 of any one or more of the gas turbine engines of theaircraft. Once received at the EEC the fuel characteristics can be usedto provide information on the fuel that is being provided from the fueltanks to the engine such that operation of the gas turbine engine(s) canbe adapted accordingly. In yet other examples, the determination module128 may be part of the sensor unit 127, which is in communication withthe EEC.

In another example, the trace substance parameter may be associated withan aromatic content of the fuel F. In such an example, the tracesubstance parameter may be a concentration (or other equivalent measureof amount) of aromatic compounds within the fuel. The inventors haveobserved that for many SAF production pathways the resulting fuelessentially has no aromatic compounds. The inventors have determinedthat this can be used as an alternative indicator of the amount of SAFcontent of the fuel. In such an example, the sensor 127 is arranged tomeasure an aromatic compound concentration in the fuel F. The fuelcharacteristic determination system 126 may, in this example, comparethe measured aromatic compound concentration to that corresponding tofossil derived fuel in order to determine a percentage of SAF in thefuel F being supplied to the aircraft. By measuring a reduction in thearomatic content compared to fossil kerosene, the relative amount of SAFpresent in the fuel as a percentage of the total fuel (e.g. thepercentage SAF per unit mass or volume) can therefore be derived. Insome examples, the trace substance parameters may indicate a measurableabsence of aromatic compounds within the fuel F, based on which the fuelcharacteristic determination module may determine that no fossilkerosene is present within the fuel F.

In order to measure an aromatic concentration in the fuel F, the sensor127 may comprise a spectroscopy device (in place of the fluorescencedevice shown in the example of FIG. 16 ). The spectroscopy device maycomprise a Fourier Transform Infra red (FT-IR) spectroscopy device or anUltraviolet Visual (UV-Vis) spectroscopy device. Such spectroscopytechniques may provide a reliable differentiation of aromatics fromparaffins and other types of hydrocarbons in an aviation fuel. Otherspectroscopy devices and techniques may however be used. Thespectroscopy device may be similar in structure to the fluorescencedevice shown in FIG. 16 .

In other examples, other trace substance parameters may be used, whichare associated with different trace substances in order to determinefuel characteristics. Appropriate sensor devices may be used accordingto the relevant trace substance to be measured. In order to determinethe one or more fuel characteristics, the determination module 128 maybe arranged to compare a measured trace substance parameter to a look-uptable of expected trace substance parameter values of fuels with knownfuel characteristics to determine the corresponding characteristics ofthe fuel F. This may allow a variety of different fuel characteristicsto be determined.

For example, the trace substance parameters may be associated with thepresence/absence/amount of a non-hydrocarbon species in the fuel,including any one or more of the following:

-   -   i) Nitrogen    -   ii) Water    -   iii) Sulphur    -   iv) Metals (including any one or more of: Al, Ca, Co, Cr, Cu,        Fe, K, Mg, Mn, Mo, Na, Ni, P, Pb, Pt, Sn, Sr, Ti, V. and/or Zn);        and/or    -   v) Halogens

The inventors have determined that generally SAF is a much purerhydrocarbon fuel with very little in the way of trace species, whencompared to conventional jet fuel processed from a crude source. As theSAF is effectively a controlled chemical process, any heteroatomicspecies seen in conventional jet fuel (sulphur, nitrogen, oxygen) ortrace metals are at very low levels (often less than the limit ofdetection by analytical techniques). Detection of thepresence/absence/concentration of one or more of these species maytherefore provide a determination of whether a fuel is SAF, or isconventional jet fuel from a fossil source.

In the examples given above the trace substances occur inherently in thefuel F. By that we mean that they are trace substances which are notartificially added to the fuel, but are present (or absent) as aconsequence of how the fuel is produced or the source that it is derivedfrom. In other examples, the trace substance(s) may be added to the fuelfor the purpose of providing an indicator for a particular fuel or fuelcharacteristic (e.g. they are a “tracer” substance). Such tracersubstances may be added deliberately for later detection by the presentmethods. These may provide a binary distinction between the fuel beingSAF or non-SAF fuel by the tracer being present or not, or the amount oftrace substance could be added in proportion to a characteristic of thefuel, for example in proportion to the SAF content or aromatic content.In such an example, the amount of tracer present may then indicate theamount of content of another component of the fuel. For example, aphosphorescent or chemiluminesent species may be used as a tracerspecies, such as phosphorous or potassium. Such tracer species may bedetected using a light or refraction sensor.

In the examples described above the fuel characteristic determinationsystem 128 is located on board the aircraft 1 so that fuel within thefuel transmission line 63 flows through it before reaching the fueltanks 53, 55. In other examples, the fuel characteristic determinationsystem 126 may be arranged to use fuel within, or sampled from, one ofthe fuel tanks 53, 55. In such an example, the sensor 127 may be locatedwithin the fuel tank so that a fluorescence or spectroscopy measurementmay be performed (or other suitable measurement depending on the tracesubstance detected). The sensor 127 may therefore be built into aninterior wall of the fuel tank such that it is exposed to fuel withinthe tank. In other examples, fuel may be sampled from the tank andpassed through the sensor 127.

In other examples, the fuel characteristic determination system 126, orat least part of it, may be located separately from the aircraft 1. Forexample, it may be included in the fuel loading line 61 so that it maydetect properties of the fuel before it reaches the aircraft. In someexamples, the sensor 127 may be located within the fuel loading line, orelsewhere separately from the aircraft 1, and arranged to transmit atrace substance parameter to the determination module 128 located onboard the aircraft 1, where it can be communicated to a control moduleof the engine (e.g. the EEC). In yet other examples, the fuelcharacteristic determination system 126 may be located entirely outsideof the aircraft. In such an example, the fuel characteristicdetermination module 128 may determine a fuel characteristic which isthen communicated to the aircraft 1 (e.g. to a control module of theengines or engines 10). In this example, a data transfer link may beprovided (e.g. a wireless or wired data connection) and may be used tocommunicate the fuel characteristics to the aircraft from the fuelcharacteristic determination system 128. In some examples, the datatransfer may be done manually by a user, e.g. a technician or otheroperator of the system may obtain the fuel characteristics from thedetermination system 126 and manually provide them to a control moduleon board the aircraft.

Fuel Characteristic Determination within Engine Using Trace SubstanceSensor:

FIG. 17 illustrates another example of the fuel characteristicdetermination system 126 described above. The fuel characteristicdetermination system 126 of FIG. 17 similarly comprises a sensor 127 anddetermination module 128 arranged to measure one or more fuelcharacteristics based on a trace substance parameter.

FIG. 17 shows a schematic view of part of the fuel system of theaircraft and the combustion equipment 16 of the gas turbine engine 10.The combustion equipment 16 comprises a plurality of fuel nozzles (notshown in FIG. 17 ) arranged to inject fuel into a combustion can. Fuelis provided to the combustion equipment 16 by a fuel delivery regulator107 under the control of the EEC 42. Fuel is delivered to the fueldelivery regulator 107 by a fuel pump 108 from a fuel source 109 onboard the aircraft 1 (e.g. one or more fuel tanks as described above).The fuel delivery regulator 107 and combustion equipment 16 may be ofknown design, and may be arranged for staged (lean-burn) combustion orrich-burn combustion.

In this example, a trace substance parameter is measured for fuel as itis being used by the engine. The sensor 127 in this example is arrangedto measure a trace substance parameter at any point within the fuelsystem of the aircraft that is upstream of the combustion equipment 16(e.g. upstream of the fuel nozzles of the combustion equipment 16) anddownstream of the fuel source 109 from which the fuel is supplied (e.g.downstream of the one or more fuel tanks 53, 55 forming the fuelsource). In some examples, the sensor 127 is located at a point withinthe engine fuel system such as in a fuel conduit within or forming partof the gas turbine engine 10 (rather than being on the aircraft 1 towhich the gas turbine engine 10 is mounted). In some examples, it islocated at a point immediately before the fuel is combusted (e.g. beforeentering the combustor). In yet other examples, the sensor 127 islocated at a point within the aircraft fuel supply system e.g. before itenters part of the gas turbine engine 10. Any of the features describedabove in connection with the example of FIG. 16 may also apply to theexample shown in FIG. 17 .

In the example shown in FIG. 17 , the trace sensor 127 is arranged suchthat all of the fuel flowing to the combustor 16 passes through it (e.g.it is arranged in series). In other examples, only some of the fuel maypass through the trace sensor 127. For example, the trace sensor may belocated in bleed line of the fuel system at which fuel is sampled fromthe fuel supply to the combustor (e.g. before or after fuel mixing).

FIG. 18 illustrates a method 1030 of determining one or more fuelcharacteristics of an aviation fuel suitable for powering a gas turbineengine of an aircraft that can be performed by the fuel characteristicdetermination systems 128 shown in FIGS. 15, 16 and 17 . The method 1030comprises: measuring 1031 one or more trace substance parameters of thefuel, the one or more trace substance parameters each associated with arespective trace substance in the fuel; and determining 1032 one or morefuel characteristics of the fuel based on the one or more tracesubstance parameters. Any of the features described above in connectionwith FIGS. 15, 16 and 17 can be incorporated in the method of FIG. 18 .

UV-Vis Spectroscopy Sensor

Fuel characteristic determination during refuelling using a UV-Vissensor Referring to FIG. 19 , another example of a fuel characteristicdetermination system 130 is illustrated, located on board the aircraft1. The fuel characteristic determination system 130 in this example isarranged to determine one or more fuel characteristics of fuel beingloaded onto the aircraft 1, those characteristics being any of thosedescribed or claimed herein. In this example, the one or more fuelcharacteristics are determined based on a measurement of thetransmittance of UV and visual spectrum (UV-Vis) light through the fuel.

Further details of the fuel characteristic determination system 130 areillustrated in FIG. 20 . The fuel characteristic determination system130 generally comprises a UV-Vis sensor 131 which is arranged to measureone or more UV transmittance parameters of the fuel being supplied tothe aircraft fuel tanks 53, 55.

The fuel characteristic determination system 130 further comprises afuel characteristic determination module 132 arranged to determine oneor more fuel characteristics of the fuel based on the transmittanceparameters. The fuel characteristic determination module 132 is arrangedto receive the transmittance parameters from the UV-Vis spectrum sensor131 such that the one or more fuel characteristics can be calculated.

In the example illustrated in FIG. 20 , the sensor 131 comprises aUV-Vis spectroscopy device arranged to measure the transmittance oflight in the UV and visual spectrum by the fuel. As can be seen in FIG.20 , fuel being loaded onto the aircraft is passed through the sensor131 where the fuel is irradiated with UV-Vis light. The sensor 131comprises a fuel inlet 131 a and a fuel outlet 131 b, and a fuel conduit131 c arranged in fluid communication between them. In the presentexample, the sensor 131 is located on board the aircraft and is arrangedto receive fuel flowing through the fuel transmission line 63 i.e. fuelflowing from the fuel line connection port 62 to the fuel tanks 53, 55.

The UV-Vis spectroscopy device comprises a light source 133 a, such asan LED, arc-lamp etc, which is arranged to emit radiation into the fuelF. In some examples, the light source 133 a may comprise two or moredifferent individual light sources arranged to provide light overdifferent wavelengths which, when combined, provide the desiredUV-Visual spectral range. The UV-Vis spectroscopy device furthercomprises a detector 133 b arranged to detect light transmitted throughthe fuel from the light source 133 a. The detector 133 b may comprise aphotodiode or similar sensor arranged to detect light emitted by thelight source 133 a. The detector 133 b is in communication with aprocessor module 133 c configured to process a signal from the detector133 c and calculate a transmittance parameter accordingly.

The transmittance parameter defines the amount of absorbance of light atspecific wavelengths of light emitted by the light source 133 a. Theinventors have determined that by measuring a UV-Vis absorption spectrumof the fuel the fuel characteristic determination module can calculatevarious characteristics of the fuel.

In one example, the one or more fuel characteristics determined by thefuel characteristic determination module 132 may include a hydrocarbondistribution of the fuel. More specifically, they may include the amountof aromatic hydrocarbon content of the fuel. In some examples, otherfuel characteristics may be directly determined from the transmittanceparameter or inferred or determined therefrom indirectly. For example,the one or more determined fuel characteristics may include thepercentage of sustainable aviation fuel in the fuel on which themeasurement is performed. As described elsewhere herein, SAF istypically associated with an essentially zero amount of aromaticcontent. The fuel characteristic determination system 130 may, in thisexample, compare the measured aromatic compound concentration to thatcorresponding to fossil derived fuel in order to determine a percentageof SAF in the fuel F being supplied to the aircraft. By measuring areduction in the aromatic content compared to fossil kerosene, therelative amount of SAF present in the fuel as a percentage of the totalfuel (e.g. the percentage SAF per unit mass or volume) can therefore bederived. In some examples, the transmittance parameters may indicate ameasurable absence of aromatic compounds within the fuel F, based onwhich the fuel characteristic determination module 132 may determinethat no fossil kerosene is present within the fuel F. In other examples,an indication that the fuel is fossil kerosene may be determined.

In order to determine the one or more fuel characteristics, thedetermination module 132 may be arranged to compare a measuredtransmittance parameter to a look-up table of expected parameter valuesof fuels with known fuel characteristics to determine the correspondingcharacteristics of the fuel to which the transmittance is measured. Thismay allow a range of fuel characteristics to be determined based on thetransmittance signature as a function of wavelength being compared tothose of known fuel types.

In the present example, the UV-Vis sensor 131 is arranged such that allof the fuel flowing through the transmission line 63 is input to thesensor inlet 131 a, flows through the fuel conduit 131 c, and is outputvia the fuel outlet 131 b to continue to the fuel tanks (i.e. the sensoris connected in series between the fuel line connection port 62 and thefuel tanks). In other examples, the sensor 131 may be connected inparallel such that only some of the fuel is redirected from thetransmission line 63, flows through the sensor, and then is returned tothe fuel transmission line 63 or to the fuel tanks. In some examples,the sensor 131 may be a microfluidic (lab-on-chip) device through whicha small sample of fuel may be passed to perform a UV-vis spectroscopymeasurement.

The determination module 132 is arranged to communicate the determinedone or more fuel characteristics to a control module of the gas turbineengine or the aircraft. In the presently described example, the one ormore fuel characteristics are transmitted to the electronic enginecontroller EEC 42. In some examples, the determination module 132 may bepart of the EEC 42 of any one or more of the gas turbine engines of theaircraft, with the control module being a sub-module of the EEC. Oncereceived at the EEC the fuel characteristics can be used to provideinformation on the fuel that is being provided from the fuel tanks tothe engine such that operation of the gas turbine engine(s) can beadapted accordingly. In yet other examples, the determination module 132may be part of the sensor unit 131, which is in communication with theEEC.

In the examples described above the fuel characteristic determinationsystem 130 is located on board the aircraft 1 so that fuel within thefuel transmission line 63 flow through it before reaching the fuel tanks53, 55. In other examples, the fuel characteristic determination system130 may be arranged to use fuel within, or sampled from, one of the fueltanks 53, 55. In such an example, the UV-Vis sensor 131 may be locatedwithin a fuel tank so that a fluorescence or spectroscopy measurementmay be performed (or other suitable measurement depending on the tracesubstance detected). The UV-Vis sensor 131 may therefore be built intoan interior wall of the fuel tank such that it is exposed to fuel withinthe tank. In other examples, fuel may be sampled from the tank andpassed through the sensor 131.

In other examples, the fuel characteristic determination system 130, orat least part of it, may be located separately from the aircraft 1. Forexample, it may be included in the fuel loading line 61 so that it maydetect properties of the fuel before it reaches the aircraft. In someexamples, the UV-Vis sensor 131 may be located within the fuel loadingline, or elsewhere separately from the aircraft 1, and arranged totransmit a transmittance parameter to a determination module 132 locatedon board the aircraft 1, where it can be communicated to a controlmodule of the engine (e.g. the EEC). In yet other examples, the fuelcharacteristic determination system 130 may be located entirely outsideof the aircraft. In such an example, the fuel characteristicdetermination module may determine a fuel characteristic which is thencommunicated to the aircraft 1 (e.g. to a control module of the enginesor engines 10). In this example, a data transfer link may be provided(e.g. a wireless or wired data connection) and may be used tocommunicate the fuel characteristics to the aircraft from the fuelcharacteristic determination system 130. In some examples, the datatransfer may be done manually by a user, e.g. a technician or otheroperator of the system may obtain the fuel characteristics from thedetermination system 130 and manually provide them to a control moduleon board the aircraft.

Fuel Characteristic Determination within the Engine Using UV-Vis Sensor

FIG. 21 illustrates another example of the fuel characteristicdetermination system 130 described above. The fuel characteristicdetermination system 130 of FIG. 21 similarly comprises a UV-Vis sensor131 and determination module 132 arranged to measure one or more fuelcharacteristics based on a transmittance parameter.

FIG. 21 shows a schematic view part of the fuel system of the aircraftand the combustion equipment 16 of the gas turbine engine 10. Thecombustion equipment 16 comprises a plurality of fuel nozzles (not shownin FIG. 21 ) arranged to inject fuel into a combustion can. Fuel isprovided to the combustion equipment 16 by a fuel delivery regulator 107under the control of the EEC 42. Fuel is delivered to the fuel deliveryregulator 107 by a fuel pump 108 from a fuel source 109 on board theaircraft 1 (e.g. one or more fuel tanks as described above). The fueldelivery regulator 107 and combustion equipment 16 may be of knowndesign, and may be arranged for staged (lean-burn) combustion orrich-burn combustion.

In this example, a transmittance parameter is measured for fuel as it isbeing used by the engine. The UV-Vis sensor 131 in this example isarranged to measure a transmittance parameter at any point within thefuel system of the aircraft that is upstream of the combustion equipment16 (e.g. upstream of the fuel nozzles of the combustion equipment 16)and downstream of the fuel source 109 from which the fuel is supplied(e.g. downstream of the one or more fuel tanks 53, 55 forming the fuelsource). In some examples, the UV-Vis sensor 131 is located at a pointwithin the engine fuel system such as in a fuel conduit within orforming part of the gas turbine engine 10 (rather than being on theaircraft 1 to which the gas turbine engine 10 is mounted). In someexamples, it is located at a point immediately before the fuel iscombusted (e.g. before it enters the combustor). It may, for example, beincluded in a bleed line of the engine fuel system (e.g. before or afterfuel mixing). In yet other examples, the UV-Vis sensor 131 is located ata point within the aircraft fuel supply system e.g. before it enterspart of the gas turbine engine 10. Any of the features described abovein connection with the example of FIG. 20 may also apply to the exampleshown in FIG. 21 .

FIG. 22 illustrates a method 1034 of determining one or more fuelcharacteristics of an aviation fuel suitable for powering a gas turbineengine of an aircraft. The method 1034 can be performed by the fuelcharacteristic determination systems 130 shown in FIGS. 19, 20 and 21 .The method 1030 comprises passing 1035 UV-visual spectrum light throughthe fuel and measuring 1036 a transmittance parameter indicating thetransmittance of light through the fuel. The method further comprisesdetermining 1037 one or more fuel characteristics of the fuel based onthe transmittance parameter. Once the fuel characteristic is determined,the method 1034 comprises communicating 1038 the one or more fuelcharacteristic to a control module of the gas turbine engine. Any of thefeatures described above in connection with FIGS. 19, 20 and 21 can beincorporated in the method of FIG. 22 .

Fuel Characteristic Determination Using Contrail Measurement

FIG. 23 illustrates another example of a fuel characteristicdetermination system 140, located on board the aircraft 1. The fuelcharacteristic determination system 140 in this example is arranged todetermine one or more fuel characteristics of fuel based on observationsof contrail formation. FIG. 23 shows the aircraft 1 in flight with eachgas turbine engine 10 producing an exhaust plume 141. A contrail 142 isalso formed in the exhaust plume 141 of each engine 10. The wing fueltanks of the aircraft 1 are not shown in FIG. 23 to aid clarity, but itis to be understood that these may still be present.

The fuel characteristic determination system 140 comprises a contrailsensor 143 and a fuel characteristic determination module 144. In thepresent example, the fuel characteristic determination system 140comprises three contrail sensors 143, one located on each wing of theaircraft 1 and one on the aircraft empennage or tail assembly. In otherexamples, only one or another number of contrail sensors may be providedas described later. The contrail sensors are each arranged to determineone or more contrail parameters related to contrail formation by the gasturbine engine 1. The contrail sensors 143 are arranged to perform asensor measurement on a region behind each gas turbine engine 10 of theaircraft 1 in which a contrail 142 is or can be formed, e.g. the exhaustplumes 141 of each engine 10.

The contrail parameters measured by the control sensors 143 include ameasured value corresponding to the degree of contrail formation takingplace within the exhaust plume 141. In some examples, this may be anindication of the presence or the absence of a contrail 142 in therespective exhaust plume 141 of each engine 10. In other examples, thecontrail parameters may be a variable indicating the relative amount ofcontrail that is being formed ranging between no contrail and a maximumdegree of contrail formation.

The contrail sensors 143 are in communication with the fuelcharacteristic determination module 144, which is arranged to receivethe contrail parameter(s) from each sensor 143. The fuel characteristicdetermination module is arranged to determine one or morecharacteristics of the fuel based on the contrail parameters received.

The inventors have observed that the presence of a contrail, or thedegree to which a contrail is formed, is dependent (at least in part) onthe characteristics of the fuel being burnt by the combustor of theengines 10. The inventors have determined that the characteristics ofthe fuel can be determined based on an active measurement of thecontrail formation in the exhaust plume of a respective gas turbineengine 10.

In one example, the one or more fuel characteristics determined based onthe measured control parameter(s) include a hydrocarbon distribution ofthe fuel. More specifically, they may include an aromatic content of thefuel. The inventors have determined that the formation of a contrail isdependent on the presence of aromatic molecules in the fuel being used.The inventors have determined that a reduction of aromatic content ofthe fuel results in an increase in the susceptibility of an engine tothe formation of contrails since the lower aromatic content of the fuelmeans that the ratio of water-vapour-to-heat added by the engine to theexhaust plume is increased. This enables the formation of contrails overa wider range of atmospheric conditions compared to fuels with a higheraromatic content such as fossil kerosene. Furthermore, less aromaticcontent may mean fewer emitted soot particles, which should (in mostsituations) lead to fewer (and hence individually larger) ice particlesin the young contrail.

In some examples, the one or more contrail parameters may includemeasurements made by the control sensors 143 over a prolonged durationof time (rather than an instantaneous observation of a contrail). Insome examples, the one or more control parameters may include aprolonged observation straddling the boundary between contrail-formingconditions and non-contrail forming conditions (or vice versa).

In some examples, other fuel characteristics as defined or claimedelsewhere herein may be determined based on the contrail parameters. Insome examples, the SAF content of the fuel may be determined based onthe contrail parameters. As the aromatic content of a SAF is typicallylower compared to fossil kerosene fuel, the resulting contrail formationwill therefore be different, and allow a determination of the SAFcontent of the fuel to be determined. In other examples, the one or morefuel characteristics determined may include an indication that the fuelis fossil kerosene. This may be determined as a result of the measuredcontrail parameters corresponding to that which would be expected iffossil kerosene were being combusted by the gas turbine engine. In otherexamples, the hydrocarbon distribution of the fuel may be determined orinferred based on the contrail parameters, for example by reference ofthe contrail parameters to those expected for a fuel with a knownhydrocarbon distribution. The fuel characteristics may be determinedusing similar comparison of parameters to those of known fuel types.

In the example illustrated in FIG. 23 , each of the contrail sensors 143comprise a contrail detector 143 a arranged to detect electromagneticradiation reflected and/or re-emitted by a contrail 142. Each contrailsensor further comprises a source of illumination 143 b arranged to emitradiation that is incident upon the contrail 142 where it is reflectedand/or re-emitted and detected by the detector 143 a. In other examples,any of the detectors 143 a may be arranged to respond to electromagneticradiation reflected and/or re-emitted by a contrail 142 in response toambient illumination (e.g. sunlight), or alternatively in response toinfra-red (or other) illumination emitted by the hot exhaust of theengine. In such examples, the sources of illumination 143 b may beabsent. The radiation detected by the detectors 143 a may be infra-redwavelength radiation emitted or reflected by a contrail. In otherexamples, other wavelengths of radiation may be detected.

The sensors 143 each have a field of view 143 c in which a signal from acontrail may be received. The sensors may be provided at any suitablelocation on the aircraft such so that the contrail forming region is inthe sensor field of view. In the present example, sensors 143 arelocated on the aircraft wings and empennage. Other sensor locations mayhowever be provided to observe each exhaust plume separately ortogether.

In yet other examples, other forms of sensor may be used to measure acontrail parameter indicative of the presence or absence of a contrail,or degree to which a contrail is formed. In some examples, the contrailsensors 143 may be arranged to detect sound returned from particles in acontrail. For example, the sources of illumination may be replaced withsources of sound (or ultrasonic) waves, which could comprise the soundgenerated by the engine in use. The detectors 143 a would then bearranged to detect the sound returned from the ice particles in acontrail.

In yet other examples, the sensors 143 may be image sensors. In thisexample, the sensors may comprise an imaging device arranged to obtainin an image of the exhaust plume (e.g. at a location where a contrailmay form behind the gas turbine engines), and determine from the imagewhether or not a contrail is formed, or the degree to which a contrailhas formed. This may be done using image processing techniques or an AI(artificial intelligence) algorithm configured to measure the size andshape of any contrail formed, or if a contrail is absent in the imagesobtained.

The fuel characteristic determination module 144 is arranged transmitthe determined one or more fuel characteristics to the electronic enginecontroller EEC 42 (not shown in FIG. 23 ). In some examples, thedetermination module 144 may be part of the EEC 42 of any one or more ofthe gas turbine engines of the aircraft. Once received at the EEC thefuel characteristics can be used to provide information on the fuel thatis being provided from the fuel tanks to the engine such that operationof the gas turbine engine(s) can be adapted accordingly. In yet otherexamples, the fuel characteristic determination module 144 may be partof one of the sensors 143, which is in communication with the EEC.

In order to determine the one or more fuel characteristics, thedetermination module 144 may be arranged to compare a measured contrailparameter to a look-up table of expected contrail parameter values offuels with known fuel characteristics to determine the correspondingcharacteristics of the fuel being used by the gas turbine engines.

In some examples, the one or more fuel characteristics are furtherdetermined by the fuel characteristic determination module 144 based onone or more an ambient atmospheric condition parameters. Each of theseatmospheric condition parameters is indicative of the ambientatmospheric conditions in which the gas turbine engines 10 are currentlyoperating. The inventors have determined that the formation of acontrail is at least partly dependent on the atmospheric conditions inwhich the respective engine is operating, in additional to thecharacteristics of the fuel being combusted. The atmospheric conditionparameters may include the ambient pressure, temperature and/or vapourpressure (humidity) in which the gas turbine engine is operating. Byfurther taking the ambient conditions into account the determination ofthe fuel characteristics may be improved.

In some examples, the ambient atmospheric condition parameters may beobtained from a sensor or sensors located on board the aircraft, whichare arranged to measure the conditions in the vicinity of the aircraft.This may provide a direct measurement of the current conditions in whichthe engine is operating. In other examples, the ambient conditionparameters may be obtained from a source of meteorological dataproviding real-time or expected information on the ambient conditions inwhich the engine is operating.

In some examples, the one or more fuel characteristics are furtherdetermined based on one or more engine or aircraft operating parameters.Various operating parameters of the aircraft or engine may have abearing on whether a contrail is formed and may therefore be taken intoaccount.

In some examples, the fuel characteristic may be calculated bydetermining the value of a varying parameter (e.g. engine operatingparameter and/or ambient condition parameter) at which a contrail isfirst formed.

In one example, the one or more fuel characteristics may be determinedbased on a contrail parameter measured during a climb phase of engineoperation. For example, the contrail parameter may indicate when acontrail first starts to form during a climb phase, and along with theambient conditions and engine operating parameter at which the contrailbegins to form, one or more characteristics of the fuel can bedetermined. This may be used to infer fuel properties, particularly theamount of water vapour released by combustion per unit of fuel energy,which in turn is related to fuel characteristics such as the hydrogenmass fraction of the fuel.

FIG. 24 illustrates a method 1040 of determining a fuel characteristicof an aviation fuel suitable for powering a gas turbine engine of anaircraft that can be performed by the fuel characteristic determinationsystem 140 shown in FIG. 23 and described above. The method 1040comprises determining 1041, during use of the gas turbine engine 10, oneor more contrail parameters. The contrail parameters are related tocontrail formation by the gas turbine engine 10 as described above.

For example, the contrail parameters may include a parameter indicativeof the degree of contrail formation taking place, or whether a contrailis present or absent. Determining 1041 the one or more contrailparameters comprises performing 1042 a sensor measurement on a region(e.g. exhaust plume 141) behind the gas turbine engine in which acontrail 142 is or can be formed. The method further comprisesdetermining 1043 one or more fuel characteristics of the fuel based onthe one or more contrail parameters as described above. Any of thefeatures described above in connection with FIG. 23 may be incorporatedinto the method of FIG. 24 .

Fuel Characteristic Determination Using Exhaust Measurement

FIG. 25 illustrates another example of a fuel characteristicdetermination system 150, located on board the aircraft 1. The fuelcharacteristic determination system 150 in this example is arranged todetermine one or more fuel characteristics of the fuel being supplied toa gas turbine engine based on measurements of an exhaust of that engine10. FIG. 25 shows the aircraft 1 in flight with each gas turbine engine10 producing an exhaust plume 141. The wing fuel tanks of the aircraft 1are not shown in FIG. 25 to aid clarity, but it is to be understood thatthese may still be present.

The determination system 150 comprises an exhaust sensor 151 and a fuelcharacteristic determination module 152. In the example shown in FIG. 25, a fuel characteristic determination system 150 is provided for eachengine, but in other examples a single fuel characteristic determinationsystem 150 could be shared between them, with exhaust sensor(s) 151 foreach engine 10 providing information to the single determination module152.

The exhaust sensor 151 is arranged to determine one or more exhaustcontent parameters of the exhaust produced by the respective gas turbineengine 10. The exhaust sensor is arranged to perform a measurement ofexhaust being produced by the engine during use. This may include atengine operating conditions corresponding to any flight phase (e.g.during cruise), or corresponding to operation while the aircraft is onthe ground (e.g. during start-up or taxi). The exhaust contentparameters may be indicative of the chemical content of the exhaustbeing produced by the engine, and may indicate the concentration,presence or absence of respective substances or species in the exhaust.In some examples, the exhaust sensor(s) 151 may be located within therespective gas turbine engine 10 so that it is arranged to perform ameasurement on exhaust gas before it is emitted by the engine. In someexamples, the exhaust sensor may perform a measurement on exhaust gasesas they flow through the engine core, or alternatively which are sampledfrom the core flow. In other examples, the exhaust sensor 151 may bearranged to perform a measurement on exhaust gases once they have exitedthe engine e.g. via the core exhaust nozzle 20 (as shown in FIG. 1 ).

The exhaust sensor 151 is in communication with the fuel characteristicdetermination module 152, which is arranged to receive the exhaustcontent parameter(s) therefrom. The fuel characteristic determinationmodule 152 is then arranged to determine one or more characteristics ofthe fuel based on the exhaust content parameters received.

The inventors have determined that characteristics of the fuel beingprovided to the gas turbine engine and combusted by its combustor can bedetermined during operation of the engine by active measurement of theproperties of the exhaust being produced. In this example, the exhaustcontent parameters include properties of the exhaust other than thepresence of a contrail.

In one example, the one or more exhaust content parameters include aparameter indicative of the nvPM content of the exhaust. The inventorshave determined that by actively measuring the nvPM content of theexhaust various characteristics of the fuel F can be determined. Bymeasuring the nvPM content, for example, a hydrogen to carbon ratio ofthe fuel F being combusted can be determined. The inventors haveobserved that a lower hydrogen to carbon ratio (e.g. compared to fossilkerosene) is associated with a higher level of nvPM production, and thedependence between them can be used to determine the characteristic ofthe fuel.

In another example, a naphthalene content can be determined based on themeasured nvPM content. In this example, a high naphthalene content (e.g.in comparison to fossil kerosene fuel) is associated with a higher levelof nvPM production. This dependence between nvPM production andnaphthalene content can be used to determine the naphthalene content ofthe fuel F.

In yet another example, an aromatic content (e.g. aromatic massfraction) of the fuel F can be determined based on the measured nvPMcontent. In this example, a high aromatic content (e.g. in comparison tofossil kerosene fuel) is associated with a higher level of nvPMproduction. This dependence between nvPM production and aromatic contentcan be used to determine the aromatic content of the fuel F.

In order to measure the nvPM content of the exhaust, the exhaust sensor151 may in one example comprise a laser induced incandescence (LII)measurement device arranged to determine the volume concentration ofnvPM in the exhaust. The LII measurement device may be arranged to firea very short, high energy pulse of a 1064 nm laser at particles withinthe exhaust. This results in the nvPM particles heating up and glowingin the visible spectrum. The peak glow is directly related to volumeconcentration of nvPM. The material density can then also be used toconvert to nvPM mass concentration.

In other examples, the exhaust sensor 151 may comprise a condensationparticle count (CPC) measurement device arranged to determine an nvPMnumber in the exhaust. The CPC device is arranged to count particlesusing laser scattering after the particles have been increased in sizeby passing them through a cloud of butanol vapour. This condenses tomake the particles large enough to count using a laser scatteringmeasurement. In this example, a volatile particle remover may berequired upstream of the CPC measurement device to remove any volatileparticles from the exhaust.

In addition, or alternatively to the nvPM content, the one or moreexhaust content parameters may include a parameter indicative of SO₂,CO₂ or CO gas within the exhaust. Based on a measurement of the SO₂content of the exhaust, the elemental sulphur content of the fuel can bedetermined by the fuel characteristics determination module. Forexample, because the SO₂ content of the exhaust will stem from oxidationof the fuel's sulphur content. Similarly, based on a measurement of CO₂or CO in the exhaust, the fuel characteristic determination module maydetermine the carbon content within the hydrocarbon content of the fuel.The carbon content can be determined based on the oxidation of thefuel's carbon content. In other examples, the exhaust content parametersbe indicative of the sulphate aerosol content of the exhaust. Theseexhaust content parameters may be used to determine a sulphur contentfuel characteristic of the fuel F.

In order to measure the SO₂, CO₂ or CO gas content of the exhaust, theexhaust sensor 151 may comprise a non-dispersive infra-red absorption(NDIR) measurement device. In order to measure the sulphate aerosol masscontent of the exhaust, the sensor 151 may comprise an aerosol massspectrometer (AMS) device. The AMS device may be arranged to directaerosols onto a hot plate (e.g. at 600° C.) and measure a resultingmolecular mass spectrum. The AMS device may be arranged to sum all ofthe spectrum peaks associated with sulphates. This method misses thesulphur nucleation peak if one exists in the cooled plume. Particle size(electrical mobility) distributions may be measured either by ScanningMobility Particle Sizer (SMPS) type measurement or Differential MobilityParticle Sizer (DMS) type measurement can determine a sulphur nucleationpeak ˜10 nm. The AMS measurement may be done using a cooled sample ofexhaust gas from the engine exhaust.

The types of exhaust sensors given above are intended to be examplesonly, and other types of sensor can used to determine nvPM and sulphurcontent of the exhaust. In yet other examples, other exhaust contentproperties may be determined, using appropriate sensors, in order forvarious other fuel characteristics to be determined (e.g. any of thosedefined elsewhere herein).

In some examples, other fuel characteristics as defined or claimedelsewhere herein may be determined based on the exhaust parameters. Insome examples, the SAF content of the fuel may be determined based onthe exhaust parameters. As, for example, the aromatic content of a SAFis typically lower compared to fossil kerosene fuel the resultingexhaust properties (e.g. nvPM content) will therefore be different, andallow a determination of the SAF content of the fuel to be made. Inother examples, the one or more fuel characteristics determined mayinclude an indication that the fuel is fossil kerosene. This may bedetermined as a result of the measured exhaust content parameterscorresponding that that which would be expected if fossil kerosene werebeing combusted by the gas turbine engine.

In order to determine the one or more fuel characteristics, thedetermination module 152 may be arranged to compare a measured exhaustparameter or parameters to a look-up table of expected exhaust parametervalues of fuels with known fuel characteristics to determine thecorresponding characteristics of the fuel being used by the gas turbineengines. For example, the measured exhaust parameters may be compared toa base-line that would be expected for a fuel of known characteristics,such as kerosene or Jet A-1 standard fuel.

In some examples, the one or more fuel characteristics are furtherdetermined by the determination module 152 based on one or more ambientatmospheric condition parameters. Each of the atmospheric conditionparameters is indicative of the ambient atmospheric conditions in whichthe gas turbine engine is currently operating. The inventors havedetermined that the exhaust properties are at least partly dependent onthe atmospheric conditions in which the engine is operating, in additionto the characteristics of the fuel being combusted. The atmosphericcondition parameters may include the ambient pressure, temperatureand/or vapour pressure (humidity) in which the gas turbine engine isoperating. By further taking the ambient conditions into account thedetermination of the fuel characteristics may be improved.

In some examples, the ambient atmospheric condition parameters may beobtained from a sensor or sensors on board the aircraft which arearranged to measure the conditions in the vicinity of the aircraft. Thismay provide a direct measurement of the current conditions in which theengine is operating. In other examples, the ambient condition parametersmay be obtained from a source of meteorological data providing real-timeor expected information on the ambient conditions in which the engine isoperating.

In some examples, the one or more fuel characteristics are furtherdetermined based on one or more engine or aircraft operating parameters.The engine operating parameters may, for example, include the powersetting of the engine.

The exhaust content parameters may be measured at any time duringoperation of the aircraft. In some examples, exhaust content parametersmay be measured during an operating stage before take-off of theaircraft so that a fuel parameter can be determined for later use duringoperation of the gas turbine for that specific flight. In some examples,the exhaust content parameters may be measured during a start-up or taxiphase of operation of the engine.

In yet other examples, the exhaust content parameters may be measured ata first engine operation condition which is associated with high levelsof emission of the relevant substance in the exhaust being produced,compared to an engine operation condition where they would be expectedto be lower. For example, if the exhaust content parameters relate tothe nvPM content of the exhaust, they may be measured during a phase ofoperation of the engine in which the nvPM production is expected to beinherently high relative to other low-nvPM emission phases. The firstengine operation condition may, for example, be a low engine power,compared to a second operating condition which may be a relativelyhigher engine power operating condition.

In some examples, the fuel characteristic determination module 152 maybase the fuel characteristic determination on a comparison of exhaustcontent parameters measured at different operating conditions of theengine. For example, the fuel characteristic determination may be basedon a comparison of exhaust content parameters measured at differentengine power levels.

FIG. 26 illustrates a method 1050 of determining a fuel characteristicof an aviation fuel suitable for powering a gas turbine engine of anaircraft that can be performed by the fuel characteristic determinationsystem 150 shown in FIG. 25 and described above. The method 1050comprises determining 1051, during use of the gas turbine engine 10, oneor more exhaust content parameters. The one or more exhaust parametersare determined by performing 1052 a sensor measurement on an exhaust ofthe gas turbine engine 10 as described above. The method 1050 furthercomprises determining 1053 one or more fuel characteristics of the fuelbased on the one or more exhaust parameters.

The method 1050 may comprise measuring the exhaust content parameters ata first engine operation condition in which emission of the respectivesubstance being measured is greater than in at a second engine operationcondition. The first engine operation condition may correspond to alower engine power compared to the second. In yet other examples, theone or more fuel characteristics may be determined based on a comparisonof exhaust content parameters determined at different engine operatingconditions (e.g. the first and second engine operating conditions).

Fuel Characteristic Determination Using Engine Performance Measurements

FIG. 27 illustrates another example of a fuel characteristicdetermination system 155, located on board the aircraft 1. The fuelcharacteristic determination system 155 in this example is arranged todetermine one or more fuel characteristics of the fuel being supplied toa gas turbine engine of the aircraft based on measurements ofperformance parameters of that engine 10.

The determination system 150 comprises a performance parameter sensor156 and a fuel characteristic determination module 157. In the exampleshown in FIG. 27 , a fuel characteristic determination system 155 isprovided for each engine, but in other examples a fuel single fuelcharacteristic determination system 155 could be shared between them,with performance parameter sensor(s) 156 for each engine 10 providinginformation to the single determination module 157.

The fuel characteristic determination module 157 is arranged todetermine one or more performance parameters of the respective gasturbine engine 10 measured during a first period of operation of theengine (e.g. during operation at a first engine operating condition).The performance parameters may be obtained from a measurement performedby the sensor 156. In the described example, the fuel characteristicdetermination module 157 is arranged to obtain the performanceparameters directly from the sensor 156. In other examples, the sensor156 may be in communication with the EEC 42 of the engine 10, and may beused as part of an existing engine control process. The fuelcharacteristic determination module 157 may, in such an example, obtainperformance parameters from the EEC 42. This may allow the fuelcharacteristic determination system 155 to make use of existing sensorsalready provided on the gas turbine engine 10. While FIG. 27 shows thefuel characteristic module 157 being separate from and in communicationwith the EEC 42, it may in some examples be part of the EEC.

The one or more fuel characteristics may be determined by the fuelcharacteristic determination module 157 after being received from thesensor 156. The fuel characteristics may, in some examples, bedetermined during a second later period of operation (e.g. duringoperation at a second engine operating condition, different from thefirst) after they have been collected during the first period ofoperation. In other examples, the determination may be made during thefirst period of operation. The inventors have determined that certainperformance parameters of the engine will have a dependence on thecharacteristics of the fuel being used, and that this can be used todetermine various fuel characteristics.

The first period of operation in which the performance parameters aremeasured may be a first flight phase, for example a take-off or climbphase. The second period of operation in which one or more fuelcharacteristics may then be determined (and optionally acted upon bycontrolling the engine accordingly) may be a second flight phase whichoccurs later in an operating mission or flight compared to the first.The second flight phase may be a cruise phase. By using performanceparameters measured during take-off or climb the engine may be operatingat an engine operating condition in which a larger variation inperformance resulting from the fuel characteristics can be observed. Forexample, during the climb phase, a variable inlet guide vane (VIGV) ofthe engine may be in a maximum open state, which would correspond to agreater observable dependence of performance on fuel characteristics.

Various engine performance parameters may be measured by the sensor(s)156. The performance parameters may be determined directly by a sensormeasurement, or may be determined indirectly from a dependence onanother parameter. In order to determine the one or more fuelcharacteristics, the fuel characteristic determination module 157 may bearranged to compare a measured performance parameter to a look-up tableof expected performance parameter values corresponding to fuels withknown fuel characteristics at the corresponding engine operatingcondition to determine the characteristics of the fuel being used. Forexample, the measured performance parameters may be compared to abase-line that would be expected for a fuel of known characteristics,such as kerosene or Jet A-1 standard fuel.

Various fuel characteristics as defined or claimed elsewhere herein maybe determined based on the performance parameters in this way. In someexamples, the SAF content of the fuel may be determined based on theperformance parameters. In other examples, the one or more fuelcharacteristics determined may include an indication that the fuel isfossil kerosene. This may be determined as a result of the measuredperformance parameters corresponding to that which would be expected iffossil kerosene were being combusted by the gas turbine engine.

In one example, the performance parameters provided to the fuelcharacteristic determination module 157 may include the rotational speedof the fan 23 and the rate of fuel being delivered to the engine. Therotational speed of the fan 23 may be determined by the N1 low pressureturbine/compressor rotation speed, which will be the same as the fanspeed 23 for a non-geared architecture, or linked to the fan speed bythe gear ratio of the gearbox 30. The variation of fan speed with fuelcharacteristics may relate to the different fuel energy per unit volumeand/or per unit mass of different types of fuel. The fuel system of theengine comprises a fuel flow meter arranged to measure the rate (e.g. bymass or by volume) at which fuel is being delivered to the engine, whichif combined with knowledge of the resulting fan speed can allow how muchbenefit the fuel is providing (i.e. more fuel energy per unit timeresults in a faster fan speed) to be determined. Based on the fan speedand fuel flow rate, the fuel characteristic determination module may bearranged to calculate how much fuel energy is being put into thecombustor per unit mass or per unit volume of fuel flow, and thus infercharacteristics of the fuel e.g. whether the fuel being provided is SAF,fossil or a percentage blend of the two (i.e. determine the percentageSAF content).

In other examples, the performance parameters provided to the fuelcharacteristics determination module 157 may include a turbine entrytemperature (TET). The TET may be as defined elsewhere herein, and mayin this example measured at the first rotor of the first turbine 17 in adownstream direction from the combustor 16 (e.g. at the highest pressureturbine). The TET may be measured directly, or more in some examples bemeasured indirectly based on a measurement further downstream in thecore airflow, for example at the second turbine 19. The inventors havedetermined that the calorific value of the fuel being burnt by thecombustor has an effect on the TET. A measurement of the TET cantherefore be used by the fuel characteristic determination module inorder to determine one or more fuel characteristics of the fuel bycomparison to the expected TET for known types of fuel. For example, theinventors have determined that an increase in fuel calorific value wouldresult in an increase in TET. This can be used as a method ofdetermining if the fuel being used is SAF, as SAF typically has a highercalorific value compared to kerosene. In some examples, a TET increaseof about 3K may be observed when using SAF compared to kerosene.

In another example, the fuel characteristic determination may be basedon a combustor fuel to air ratio. This may be determined by the mass offuel flow to the combustor in comparison to the core air flow. Theinventors have determined that this ratio would decrease with use of afuel such as SAF, and may therefore provide another means of determiningthat SAF is being used by the engine.

In yet other examples, other engine performance parameters may be used,including HP spool speed, T30 and/or T40 (as defined elsewhere herein).In other examples, the fuel flow rate required to achieve a desired fanspeed at the current ambient conditions and aircraft forward speed(airspeed) may be used to determine the characteristics of the fuelbeing supplied to the engine. For SAF the required fuel flow rate toachieve a given fan speed will be lower (with respect to mass) or higher(with respect to volume) than the required fossil fuel flow rate toachieve the same fan speed at the same operating conditions. Similarcomparisons between measured performance parameters and those expectedfor known fuel characteristics can be used to determine a variety offuel characteristics.

The fuel characteristic determination module 157 may be arranged todetermine each fuel characteristic based on a plurality of differentengine performance parameters. The plurality of performance parametersmay include at least two different performance parameters, andpreferably at least three different performance parameters. The fuelcharacteristic may be determined based on a comparison of thosedifferent performance parameters to each other. By using more than oneperformance parameter the accuracy or reliability of the fuelcharacteristic determination may be improved.

In response to the fuel characteristic(s) determined by the fuelcharacteristics determination module, the operation of the gas turbineengine (or the aircraft) may be adjusted or modified accordingly duringthe second time period of operation. For example, engine performanceparameters obtained during the take-off or climb phase may be acted uponduring cruise operation or during descent.

In the present example, in which the aircraft comprises fuel tanks whichmay be configured to store fuel having different fuel characteristics,the gas turbine engine is only operated according to the one or morefuel characteristics during the second time period if fuel is being usedfrom the same fuel tank, or fuel known to have the same fuelcharacteristics, as during the first time period. This means that theengine operation is changed only if the same fuel is being used so thatthe change in response to the fuel is suitable.

In one example, a fuel burn parameter or characteristic of the enginemay be controlled based on the determined fuel characteristic. Forexample, the engine may be controlled during the cruise phase ofoperation from a fuel burn perspective based on the fuel characteristicsdetermined from take-off/climb phase performance parameters. In otherexamples, the N1 speed of rotation of the high pressureturbine/compressor may be modified in response to the one or more fuelcharacteristics. For example, the N1 rotation speed may be reducedduring climb so that the resulting TET matched the temperaturecorresponding to that when operating on kerosene (in such an example,the first flight phase may be take-off, and the second a climb phase).In other examples, the N1 rotation speed during cruise may be modifiedin response to the fuel characteristics determined during take-off. Inyet another example, the N1 rotation speed may be modified during adescent flight phase based on the determined fuel characteristics, againby modifying the N1 rotation speed so that it corresponds to that whichwould be expected should the engine be running on kerosene. The N1rotation speed in these examples may be modified by changing thecorresponding rating tables in the engine control unit (e.g. the EEC42).

FIG. 28 illustrates a method 1058 of determining one or more fuelcharacteristics of an aviation fuel suitable for powering a gas turbineengine of an aircraft that can be performed by the fuel characteristicdetermination system 155 shown in FIG. 27 and described above. Themethod 1058 comprises determining 1059 one or more performanceparameters of the gas turbine engine measured during a first time periodof operation of the gas turbine engine. Once the performance parametersare determined, the method comprises determining 1060, one or more fuelcharacteristics of the fuel based on the one or more performanceparameters. In some examples, the one or more fuel characteristics aredetermined during a second period of operation as described above, ormay be determined during the first period of operation.

In some examples, the method 1058 is part of a method of operating anaircraft having the gas turbine engine (e.g. method 1065 describedbelow), and so may include a step 1061 of operating the gas turbineengine or the aircraft according to the one or more fuel characteristicsduring the second time period of operation. Operation of the aircraft orthe gas turbine engine may include modifying a control parameter inresponse to the determined fuel characteristics as described below.

Any of the features described above in connection with the fuelcharacteristic determination system 157 in reference to the exampleshown in FIG. 27 may be incorporated into the method of FIG. 28 .

Aircraft Operation According to a Fuel Characteristic or Parameter onwhich a Fuel Characteristic Determination is Based

The fuel characteristics determined using any of the fuel characteristicdetermination systems or methods of determining a fuel characteristic inthe examples herein may be used in the operation of the aircraft, andmore specifically operation of the gas turbine engine(s) of theaircraft. This may allow the operation of the aircraft 1 to be modifiedin response to the fuel characteristic determined.

The present application therefore further provides a method 1065 ofoperating an aircraft 1 powered by one or more gas turbine engines 10 asillustrated in FIG. 29 . The method 1065 may be a method of operatingthe aircraft 1 of any of the examples described herein. The method 1065comprises determining 1066 one or more fuel characteristics. This maycomprise using any of the methods described herein. The method 1065further comprises operating 1067 the aircraft 1 according to the one ormore fuel characteristics. Operating the aircraft 1067 may morespecifically comprise operating the gas turbine engine(s) 10 mounted tothe aircraft 1, but may include operating other parts of the aircraft.

Once one or more fuel characteristics are known, the gas turbine engine10 or the aircraft more generally may be controlled or operated invarious different ways to take advantage of that knowledge. The step ofoperating 1067 the gas turbine engine or the aircraft may comprisemodifying 1067 a a control parameter of the aircraft, and specifically acontrol parameter of the gas turbine engine, in response to the one ormore fuel characteristics. Modifying the control parameter may includeany one or more of the following:

-   -   i) Modifying a control parameter of a heat management system of        the gas turbine engine (e.g. a fuel-oil heat exchanger 118)        based on the one or more fuel characteristics. By modifying the        operation of the heat exchanger 118 the temperature of fuel        supplied to the combustor 16 of the engine 10 can be changed. In        one example, modifying the operation of the heat management        system or changing the temperature of the fuel may comprise        increasing the temperature of the fuel if the fuel        characteristics indicate that the fuel can tolerate operating at        a higher temperature without risk of coking or thermal        breakdown.    -   ii) When more than one fuel is stored aboard an aircraft 1,        modifying a control parameter that controls a selection of which        fuel to use for which operations (e.g. for ground-based        operations as opposed to flight, for low-temperature start-up,        or for operations with different thrust demands) based on fuel        characteristics such as % SAF, nVPM generation potential,        viscosity, and calorific value. A fuel delivery system of the        aircraft may therefore be controlled appropriately based on the        fuel characteristics. The fuel delivery system may be controlled        to supply the engine with fuel having a different fuel        characteristic to that measured. This may include, for example,        providing fuel with a relatively lower aromatic content;        providing fuel with a lower SAF content; or providing fossil        Kerosene fuel. The fuel supply may be controlled by switching        between fuel tanks, or changing a fuel blend ratio.    -   iii) Modifying a control parameter to adjust one or more flight        control surfaces of the aircraft 1, so as to change route and/or        altitude based on knowledge of the fuel.    -   iv) Modifying a control parameter to modify the spill percentage        of a fuel pump (i.e. the proportion of pumped fuel recirculated        instead of being passed to the combustor) of a fuel system of        the aircraft according to the one or more fuel characteristics,        for example based on the % SAF of the fuel. The pump and/or one        or more valves may therefore be controlled appropriately based        on the fuel characteristics.    -   v) Modifying a control parameter to change the scheduling of        variable-inlet guide vanes (VIGVs) based on fuel        characteristics. The VIGVs may be moved, or a movement of the        VIGVs be cancelled, as appropriate based on the fuel        characteristics.

In the examples above, the gas turbine engine or the aircraft isoperated according to the one or more fuel characteristics by makingchanges to how the aircraft or gas turbine engine are controlled duringtheir use. This may be done, for example, by a control system of theengine (such as the EEC 42) making changes to various control parametersof the engine. Similar changes may be implemented by other controlsystems of the aircraft during use (e.g. during flight). The EEC may bemore generally referred to as an example of a control system 42 arrangedto control operation of the aircraft (e.g. it may be a control module ofa control system).

The present application further provides an aircraft 1 having a fuelcharacteristic determination system according to any one or more of theexamples disclosed or claimed herein. The aircraft further comprises acontrol system arranged to control operation of the aircraft accordingto one or more fuel characteristics determined by the fuelcharacteristics determination system. The control system may comprisethe engine EEC 42, with which the fuel characteristic determinationsystem may be in communication or partly integrated therein. In otherexamples, other control systems of the aircraft may be provided withfuel characteristics and the aircraft controlled accordingly. Thecontrol system may be arranged to control operation of the aircraftaccording to a parameter on which a fuel characteristic determination isbased directly, rather than requiring a fuel characteristic to bedetermined as described below.

The step of operating 1067 the gas turbine engine or aircraft accordingto the one or more fuel characteristics may be performed automaticallyin response to the determination of fuel properties without anyintervention of the pilot. In some examples, it may be performed afterapproval by a pilot, following the pilot being notified of a proposedchange. In some examples, the step 1067 a may include automaticallymaking some changes, and requesting others, depending on the nature ofthe change. In particular, changes which are “transparent” to thepilot—such as internal changes within engine flows which do not affectengine power output and would not be noticed by a pilot—may be madeautomatically, whereas any changes which the pilot would notice may benotified to the pilot (i.e. a notification appearing that the changewill happen unless the pilot directs otherwise) or suggested to thepilot (i.e. the change will not happen without positive input from thepilot). In implementations in which a notification or suggestion isprovided to a pilot, this may be provided on a cockpit display of theaircraft, and/or sent to a separate device such as a portable tablet orother computing device, and/or announced via audible sound such assynthesized speech or recorded message or a particular tone indicativeof the proposed/notified change.

In other examples, the step of operating 1067 the gas turbine engineaccording to the one or more fuel characteristics may include providing1067 b the gas turbine engine with fuel having different characteristicsto that of the fuel for which the one or more fuel characteristics weremeasured in step 1066. This provision of a different fuel may includeloading fuel having different fuel characteristics into the fuel tanksof the aircraft when refuelling the aircraft.

In some examples, operation of the aircraft may be modified in responseto one or more of the parameters disclosed herein upon which the fuelcharacteristics are determined. This may include, for example, thevibration parameter, swell parameter, trace substance parameter, UV-Vistransmittance parameter, contrail parameter, exhaust parameter, andengine performance parameter. The aircraft may therefore be controlledbased on such parameters, without a fuel characteristic necessarily alsobeing calculated.

In one such example, the operation of the aircraft may be modified inresponse to the one or more contrail parameters, without a fuelcharacteristic being determined. An example of such a method is shown inFIG. 30 . FIG. 30 illustrates a method 1070 of operating an aircraft 1having a gas turbine engine 10. The method 1070 comprises: determining1071, during use of the gas turbine engine 10, one or more contrailparameters related to contrail formation by the gas turbine engine 10.Determining 1071 the one or more contrail parameters comprisesperforming 1073 a sensor measurement on a region behind the gas turbineengine in which a contrail is or can be formed as described above inconnection with the example shown in FIG. 23 . The contrail parametersare determined in step 1071 during varying operation of the aircraft(e.g. during a period of varying engine operating parameter and/orambient condition parameter). The contrail parameters determined mayindicate the value of a varying parameter at which a contrail is firstformed. Once the one or more contrail parameters are determined in thisway, the method 1070 further comprises controlling 1074 an operatingparameter of the aircraft according to the one or more contrailparameters and the value of the varying parameter to which theycorrespond. As discussed above, this may involve measuring the engineperformance parameters and/or ambient conditions at which contrailformation begins during a climb phase of the aircraft operation. Thecontrol of the aircraft may additionally or alternatively include any ofthe examples of control of the aircraft in response to fuelcharacteristics described above.

Another example in which the aircraft is controlled based on a sensorparameter rather than a fuel characteristic is illustrated in FIG. 31 .In this example, steps 1026, 1027 and 1028 of method 1025 shown in FIG.14 are incorporated into a method of operating an aircraft. In thisexample therefore, FIG. 31 illustrates a method of operating an aircraft1090 which comprises measuring 1091 a swell parameter of a seal material(which is the same material as other seals 125 provided on the aircraft1) using the steps of method 1025. The method 1090 further comprisesoperating 1092 the aircraft according to the swell parameter. Operating1092 the aircraft according to the swell parameter may include providing1093 the one or more gas turbine engines with fuel having a differentcharacteristic compared to the fuel for which the swell parameter wasmeasured. The fuel having a different characteristic may be provided byrefuelling the aircraft with fuel having a different characteristic tothat already in its fuel tank(s), or by suppling fuel from a differentfuel tank onboard the aircraft which holds fuel having a differentcharacteristic. Suppling fuel from a different fuel source on board theaircraft may comprise altering a blend of fuels from different fuelsources, or switching between fuel having different characteristics.Providing fuel having a different characteristic may comprise any one ormore of: i) providing fuel with a relatively higher aromatic content;ii) providing fuel with a lower SAF content; iii) providing kerosene.This may allow seal swell to be increased if operation using the currentfuel is determined to provide inadequate seal swell, which could lead toa reduction in seal performance.

In the previously described examples, the various fuel characteristicdetermination systems are arranged to determine the one or more fuelcharacteristics based only on the respective parameter described in eachexample (e.g. on only one of a vibration parameter, swell parameter,trace substance parameter, UV-Vis transmittance parameter, contrailparameter, exhaust parameter, or engine performance parameter) In otherexamples, any of the fuel characteristic determination modules ormethods of determining one or more fuel characteristics described orclaimed herein may be arranged to base the fuel characteristics on anyone or more of the parameters described herein i.e. any one or more ofthe vibration parameter, swell parameter, trace substance parameter,UV-Vis transmittance parameter, contrail parameter, exhaust parameter,and engine performance parameter. This may allow a greater range ortypes of fuel characteristic to be determined, or may improve theaccuracy or reliability of the fuel characteristic determination.

In any of the examples described herein, characteristics of fuel as itis being loaded onto the aircraft may be determined (e.g. as shown inthe examples of FIGS. 5, 9, 13, 15 and 19 ). In such examples, the oneor more fuel characteristics determined may be communicated to the EEC42 directly if it is running during refuelling, or may otherwise storedand communicated to the EEC when it is activated. If the EEC is notactive when fuel characteristics are determined, they may becommunicated to another control system of the aircraft.

Where fuel characteristics are determined for fuel being loaded onto theaircraft that fuel may be mixed with fuel already present in the fueltanks (e.g. from previous flights). The determined fuel characteristicsmay therefore be combined with those determined from previous times atwhich the aircraft was refuelled in order to determine thecharacteristics of the fuel stored in the aircraft fuel tanks. This maybe done using a summing method in which the amount of fuel loaded intothe tanks, the amount of fuel used during each flight, and thecorresponding characteristics of the fuel loaded are logged and combinedto determine the fuel characteristics of fuel actually stored within theaircraft tanks at a given time.

Maintenance Schedule Generation According to Fuel Characteristics

The one or more fuel characteristics determined using any of the methodsdescribed or claimed herein may be used in the generation of amaintenance schedule for the respective gas turbine engine, or moregenerally the aircraft on which the gas turbine engine is mounted.

FIG. 32 illustrates an example of a method 1080 of generating amaintenance schedule for an aircraft. The aircraft comprises one or moregas turbine engines, and may be the aircraft 1 described in respect toany of the other examples herein. The method 1080 comprises determining1081 one or more fuel characteristics of a fuel with which the gasturbine engine has been, or is to be, powered. The one or more fuelcharacteristics may be those of a fuel or fuels used during previoususes of the gas turbine engine, or fuel held within the fuel tanks ofthe aircraft. The one or more fuel characteristics may be determinedusing any of the methods described herein, but is not limited to onlythose methods in other examples. The fuel characteristics may bedetermined as described herein and provided automatically to amaintenance schedule determination module without human intervention. Insome other examples, determining the one or more fuel characteristicsmay involve them being input manually to a maintenance scheduledetermination module, e.g. during re-fuelling of the aircraft. This mayinclude reading the fuel characteristics from a fuel specification, orfrom the output of a fuel characteristic determination system, andinputting them manually by a human user to the maintenance scheduledetermination module. The method 1080 further comprises generating 1082maintenance schedule according to the one or more fuel characteristics.In other examples, one or more maintenance schedules (e.g. one for eachengine) may be determined.

The inventors have determined that the characteristics of the fuel thathas been used to power the gas turbine have an effect on the operationof the gas turbine engine and the aircraft in general and so may requirea change in a maintenance schedule for that gas turbine engine oraircraft. The change in the maintenance schedule may include a change tothe maintenance operations scheduled to take place, and/or a change tothe time/frequency at which maintenance operations are performed. Insome examples, generation of a maintenance schedule may include themodification of an existing schedule in response to the fuelcharacteristics, or the generation of a new one.

The step of generating 1082 the maintenance schedule may comprisecomparing 1083 the one or more determined fuel characteristics to anexpected fuel characteristic, and modifying 1084 a maintenance scheduleaccordingly. An existing pre-defined maintenance schedule for a gasturbine engine, or aircraft in general, may be associated with anexpected fuel characteristic. For example, the maintenance schedule maybe determined according to a specified type of fuel to be used by theaircraft so that maintenance can be performed based on how the engine isexpected to operate using that fuel. The existing maintenance schedulemay be modified in response to determining that a deviation from theexpected fuel characteristics has occurred. This may allow themaintenance schedule to be tailored to the actual fuel that has beenused, rather than by assuming that the specified fuel has been used.

In one example, the existing maintenance schedule may specify operationof the aircraft 1 using a fuel having a specific SAF content, forexample, a SAF rich fuel having a certain proportion of SAF compared tofossil kerosene, or require 100% SAF fuel to be used. Deviation from useof fuel with the specified SAF content as indicated by the determinedfuel characteristics may result in a modification to the maintenanceschedule to account for operation outside of specification. In otherexamples, the modification of the maintenance schedule may be based onother characteristics of the fuel as defined elsewhere herein. Forexample, the aromatic content of the fuel, or an indication that fossilkerosene has been used, may be determined and the maintenance schedulemodified or otherwise generated accordingly.

In some examples, a periodic measurement of the fuel characteristics maybe performed on which the maintenance schedule determination is based.This may allow the maintenance schedule to be generated based on aneffect on the performance of the engine or aircraft that may occur overa prolonged period of use with fuel having certain properties. This mayallow slow changes in the performance of the engine to be taken intoaccount, rather than using a “real-time” modification of the maintenanceschedule based on only a single real-time fuel characteristicdetermination. For example, the one or more fuel characteristicsdetermined in step 1081 may indicate that a threshold level of fuelcoking or thermal breakdown has taken place. This may lead to surfacedeposit build up within components of the gas turbine engine over time(e.g. fuel nozzles), and may require more frequent maintenance orsusceptible components to be replaced or cleaned. By using periodicdetermination of the fuel characteristics the level of coking may bemonitored over time and the maintenance schedule adapted accordingly.

In some examples, determining the one or more fuel characteristics maycomprise measuring a change in the properties of a sensor componentwhich is exposed to the fuel used to power the engines 10 of theaircraft. The sensor component may in some examples be a piezoelectriccrystal which is exposed to fuel loaded onto the aircraft or being usedby the gas turbine engine as discussed in connection with the examplesshown in FIGS. 5 to 8 . In this example, the method 1080 may thereforemake use of the fuel determination system 114 described in the examplesabove. Determining the one or more fuel characteristics in such anexample may comprise measuring a vibration mode of the piezoelectriccrystal, which provides an indication of a surface deposit formed on thecrystal. The maintenance schedule may be modified based on the detectionof such a surface deposit indicating that fuel having a characteristiccausing surface deposits to form with the engine or fuel system has beenused, and maintenance is to be carried out accordingly. The maintenanceschedule may be modified according to a threshold level of coking orfuel thermal breakdown having been exceeded. In such an example, thefuel characteristic may be an indication that the fuel has caused asurface deposit, rather than requiring a further determination of a fuelcharacteristic that is associated with such surface deposit formationbeing found. This may allow the maintenance schedule to be modifiedbased on a surface deposit formed, regardless of by what mechanism itwas formed.

In other examples, the method 1081 may make use of the detection device120 illustrated in FIGS. 10, 11 and 13 and described above. In thisexample, the sensor component which is exposed to fuel and on which fuelcharacteristic determination is based comprises a seal material 121.This may be a seal material which is the same as one or more seals usedwithin the fuel system of the gas turbine engine 10 as described abovein connection with FIGS. 13 and 14 . As discussed above, one or morefuel characteristics may be determined based on a swell parameter of theseal material. The inventors have determined that the one or more fuelcharacteristics determined in this way may be used to generate amaintenance schedule according to the effect the fuel characteristicswill have on seals that are exposed to fuel on board the aircraft (e.g.in the fuel system arranged to store and supply fuel to the gas turbineengine, and within the engine itself). In this example, the one or morefuel characteristics on which the maintenance schedule is generatedindicate whether a threshold level of swell of the seal material hasoccurred. If the swell has not exceeded a predetermined threshold, thismay indicate that sealing performance may have been inhibited and themaintenance schedule should be generated or modified accordingly.

The method 1080 of generating a maintenance schedule for an aircraft maybe part of a method of maintaining an aircraft. An example of such amethod 1085 is illustrated in FIG. 33 . The method 1085 of maintainingan aircraft comprises generating 1086 a maintenance schedule using themethod 1080 described above. Once the maintenance schedule has beengenerated, the method 1085 comprises performing 1087 maintenance on theaircraft according to the maintenance schedule. Performing maintenancemay comprise performing maintenance on the gas turbine engine(s) 10, oron the aircraft more generally. The step of performing maintenance maycomprise steps taken by a technician in response to the generatedmaintenance schedule. In other examples, maintenance steps may beperformed automatically without the intervention of a technician. Forexample, software updates or changes to control programs (e.g. those ofthe EEC) may be performed automatically during maintenance processwithout human intervention.

FIG. 34 illustrates an example of an aircraft 1 having a maintenanceschedule generation system 160 arranged to carry out the method 1085 ofFIG. 32 . The maintenance schedule generation system 160 in this examplecomprises a fuel characteristic determination module 117 correspondingto the example described above, in which fuel characteristics aredetermined based on a vibrational parameter of a piezoelectric crystal(e.g. using sensor 115 which comprises a piezoelectric crystal 116 asdescribed above). The fuel characteristic determination module mayhowever be any one or more of those disclosed or claimed elsewhereherein.

The maintenance schedule generation system 160 further comprises amaintenance schedule generation module 162 in communication with thefuel characteristic determination module 117 and configured to generatea maintenance schedule according to the one or more fuel characteristicsreceived therefrom. The maintenance schedule generation module 162 maybe configured to generate the maintenance schedule as described above.

The maintenance schedule generation system 160 may be located on boardthe aircraft 1 as shown in the example of FIG. 34 . In this example, aseparate maintenance schedule generation system 160 is provided for eachengine 10. In other examples, a single system may be provided, forexample having a single generation module 162 configured to receive fuelcharacteristics from a single fuel characteristic determination moduleon board the aircraft, or from separate fuel characteristicdetermination modules provided for each engine. A single maintenanceschedule for the aircraft 1 as a whole may be produced, or separatemaintenance schedules for each engine 10 produced accordingly.

In other examples, the maintenance generation system 160 may be locatedat least partly outside of the aircraft 1. For example, the maintenanceschedule generation module 162 may be located separately from theaircraft 1, and may be configured to communicate via a wired or wirelessdata link connection with a fuel characteristic determination module sothat the one or more fuel characteristics may be received and amaintenance schedule generated accordingly.

The maintenance schedule generation module 162 may be arranged to outputa maintenance schedule to a technician performing maintenance on theaircraft 1, or to an aircraft health monitoring system arranged tomanage the maintenance of the aircraft 1 (or engines 10 separately). Insome examples, the maintenance generation module 162 may be incommunication with the EEC 42 so that updates or reconfiguration ofcontrol parameters stored in the EEC may be carried out (e.g.automatically).

It will be understood that the invention is not limited to the examplesabove-described and various modifications and improvements can be madewithout departing from the concepts described herein. Except wheremutually exclusive, any of the features may be employed separately or incombination with any other features and the disclosure extends to andincludes all combinations and sub-combinations of one or more featuresdescribed herein.

We claim:
 1. A method of determining one or more fuel characteristic ofan aviation fuel suitable for powering a gas turbine engine of anaircraft, the method comprising: determining, during use of the gasturbine engine, one or more contrail parameters related to contrailformation by the gas turbine engine, wherein determining the one or morecontrail parameters comprises performing a sensor measurement on aregion behind the gas turbine engine in which a contrail is or can beformed; and determining one or more fuel characteristics of the fuelbased on the one or more contrail parameters.
 2. The method of claim 1,wherein the one or more contrail parameters include a parameterindicative of the degree of contrail formation taking place or thepresence or absence of a contrail produced by the gas turbine engine. 3.The method of claim 1, wherein determining the one or more contrailparameters comprises measuring electromagnetic radiation reflectedand/or re-emitted by a contrail.
 4. The method of claim 1, whereindetermining the one or more contrail parameters comprises detecting thepresence or absence of a contrail, or the degree to which a contrail isformed, in an image of the region behind the gas turbine engine.
 5. Themethod of claim 1, wherein the one or more fuel characteristics arefurther determined based on one or more ambient atmospheric conditionparameters, each indicative of the ambient atmospheric conditions inwhich the gas turbine engine is currently operating.
 6. The method ofclaim 5, further comprising obtaining the one or more ambientatmospheric conditions from a source of meteorological data providingreal-time or expected information on the ambient atmospheric conditions.7. The method of claim 5, further comprising obtaining the one or moreambient atmospheric conditions from a sensor arranged to measure theambient conditions in the vicinity of the aircraft.
 8. The method ofclaim 1, wherein the one or more fuel characteristics are furtherdetermined based on one or more engine or aircraft operating parameters.9. The method of claim 8, wherein the one or more fuel characteristicsare determined based on measuring the value of a varying parameter atwhich contrail formation begins.
 10. The method of claim 1, wherein theone or more fuel characteristics determined include any one or more of:(i) a hydrocarbon distribution of the fuel; (ii) a percentage ofsustainable aviation fuel in the fuel; (iii) an aromatic hydrocarboncontent of the fuel; and/or (iv) an indication that the fuel is a fossilfuel e.g. kerosene.
 11. A fuel characteristic determination system fordetermining one or more fuel characteristic of an aviation fuel suitablefor powering a gas turbine engine of an aircraft, the system comprising:a contrail sensor arranged to determine one or more contrail parametersrelated to contrail formation by the gas turbine engine, the contrailsensor being arranged to perform a sensor measurement on a region behindthe gas turbine engine in which a contrail is or can be formed; and afuel characteristic determination module arranged to determine one ormore fuel characteristics of the fuel based on the one or more contrailparameters.
 12. The fuel characteristic determination system of claim11, wherein the one or more contrail parameters include a parameterindicative of the degree of contrail formation taking place or thepresence or absence of a contrail produced by the gas turbine engine.13. The fuel characteristic determination system of claim 11, whereinthe contrail sensor is arranged to measure electromagnetic radiationreflected and/or re-emitted by a contrail.
 14. The fuel characteristicdetermination system of claim 11, wherein the contrail sensor isarranged to detect the presence or absence of a contrail, or the degreeto which a contrail is formed, in an image of the region behind the gasturbine engine.
 15. The fuel characteristic determination system ofclaim 11, wherein the fuel characteristic determination module isfurther arranged to determine the one or more fuel characteristics basedon one or more ambient atmospheric condition parameters, each indicativeof the ambient atmospheric conditions in which the gas turbine engine iscurrently operating, and optionally the fuel characteristicdetermination module is further arranged to: a) obtain the one or moreambient atmospheric conditions from a source of meteorological dataproviding real-time or expected information on the ambient atmosphericconditions; and/or b) obtain the one or more ambient atmosphericconditions from a sensor arranged to measure the ambient conditions inthe vicinity of the aircraft.
 16. The fuel characteristic determinationsystem of claim 11, wherein the fuel characteristic module is furtherarranged to determine the one or more fuel characteristics based on oneor more engine or aircraft operating parameters, and optionally: whereinthe one or more fuel characteristics are determined based on measuringthe value of a varying parameter at which contrail formation begins. 17.The fuel characteristic determination system of claim 11, wherein theone or more fuel characteristics determined include any one or more of:(i) a hydrocarbon distribution of the fuel; (ii) a percentage ofsustainable aviation fuel in the fuel; (iii) an aromatic hydrocarboncontent of the fuel; and/or (iv) an indication that the fuel is a fossilfuel e.g. kerosene.
 18. A method of operating an aircraft having a gasturbine engine, the method comprising: determining one or more fuelcharacteristics using the method of claim 1; and operating the aircraftaccording to the one or more fuel characteristics.
 19. The method ofclaim 18, wherein operating the aircraft according to the one or morefuel characteristics comprises any one or more of: i) changing anoperating parameter of a heat management system of the gas turbineengine; ii) changing a fuel temperature of the fuel within the gasturbine engine; and/or iii) changing a flight characteristic of theaircraft.
 20. A method of operating an aircraft having a gas turbineengine, the method comprising: determining, during use of the gasturbine engine, one or more contrail parameters related to contrailformation by the gas turbine engine, wherein determining the one or morecontrail parameters comprises performing a sensor measurement on aregion behind the gas turbine engine in which a contrail is or can beformed, and wherein the one or more control parameters are determinedduring a varying operation of the aircraft and correspond to a value ofa varying parameter at which a control begins to form; and controllingthe operating parameter of the aircraft or gas turbine engine accordingto the one or more contrail parameters.